diff --git a/src/function/ArgumentValidator.java b/src/function/ArgumentValidator.java
index 7cd5ffc..a79f1b3 100644
--- a/src/function/ArgumentValidator.java
+++ b/src/function/ArgumentValidator.java
@@ -65,10 +65,10 @@ public class ArgumentValidator {
     private void validateListNotDotted(Cons argumentList) {
         SExpression next = argumentList.getRest();
 
-        for (Cons current = argumentList; next.consp(); next = current.getRest())
+        for (Cons current = argumentList; next.isCons(); next = current.getRest())
             current = (Cons) next;
 
-        if (!next.nullp())
+        if (!next.isNull())
             throw new DottedArgumentListException(functionName, argumentList);
     }
 
@@ -114,7 +114,7 @@ public class ArgumentValidator {
     }
 
     private boolean isFirstArgumentValid(Cons argumentList) {
-        return argumentList.nullp() || isExpectedFirstArgumentType(argumentList.getFirst());
+        return argumentList.isNull() || isExpectedFirstArgumentType(argumentList.getFirst());
     }
 
     private boolean isExpectedFirstArgumentType(SExpression firstArgument) {
@@ -122,11 +122,11 @@ public class ArgumentValidator {
     }
 
     private boolean isDisallowedNil(SExpression argument) {
-        return !isNilAcceptable && argument.nullp();
+        return !isNilAcceptable && argument.isNull();
     }
 
     private void validateTrailingArguments(Cons argumentList) {
-        for (Cons cons = (Cons) argumentList.getRest(); !cons.nullp(); cons = (Cons) cons.getRest())
+        for (Cons cons = (Cons) argumentList.getRest(); !cons.isNull(); cons = (Cons) cons.getRest())
             if (!isExpectedTrailingArgumentType(cons.getFirst()))
                 throw new BadArgumentTypeException(functionName, cons.getFirst(), trailingArgumentType);
     }
diff --git a/src/function/LispFunction.java b/src/function/LispFunction.java
index b8a057c..fe6a27f 100644
--- a/src/function/LispFunction.java
+++ b/src/function/LispFunction.java
@@ -6,11 +6,6 @@ public abstract class LispFunction {
 
     public abstract SExpression call(Cons argList);
 
-    /**
-     * Determine if the arguments passed to this Lisp function should be evaluated. A subclass
-     * should override this method to return false if it does not want its arguments to be evaluated
-     * prior to being passed.
-     */
     public boolean evaluateArguments() {
         return true;
     }
diff --git a/src/function/LispSpecialFunction.java b/src/function/LispSpecialFunction.java
new file mode 100644
index 0000000..5bf3c43
--- /dev/null
+++ b/src/function/LispSpecialFunction.java
@@ -0,0 +1,10 @@
+package function;
+
+public abstract class LispSpecialFunction extends LispFunction {
+
+    @Override
+    public boolean evaluateArguments() {
+        return false;
+    }
+
+}
diff --git a/src/function/UserDefinedFunction.java b/src/function/UserDefinedFunction.java
index c14a6ca..6648453 100644
--- a/src/function/UserDefinedFunction.java
+++ b/src/function/UserDefinedFunction.java
@@ -24,7 +24,7 @@ public class UserDefinedFunction extends LispFunction {
         this.executionContext = ExecutionContext.getInstance();
         this.functionScope = executionContext.getScope();
 
-        for (this.formalParameters = new ArrayList<>(); lambdaList.consp(); lambdaList = (Cons) lambdaList.getRest())
+        for (this.formalParameters = new ArrayList<>(); lambdaList.isCons(); lambdaList = (Cons) lambdaList.getRest())
             this.formalParameters.add(lambdaList.getFirst().toString());
 
         this.argumentValidator = new ArgumentValidator(this.name);
@@ -38,10 +38,7 @@ public class UserDefinedFunction extends LispFunction {
         SymbolTable callingScope = executionContext.getScope();
         executionContext.setScope(functionScope);
 
-        SExpression lastEvaluation = Nil.getInstance();
-
-        for (Cons expression = body; expression.consp(); expression = (Cons) expression.getRest())
-            lastEvaluation = eval(expression.getFirst());
+        SExpression lastEvaluation = evaluateBody();
 
         executionContext.setScope(callingScope);
         releaseParameterValues();
@@ -59,6 +56,15 @@ public class UserDefinedFunction extends LispFunction {
         }
     }
 
+    private SExpression evaluateBody() {
+        SExpression lastEvaluation = Nil.getInstance();
+
+        for (Cons expression = body; expression.isCons(); expression = (Cons) expression.getRest())
+            lastEvaluation = eval(expression.getFirst());
+
+        return lastEvaluation;
+    }
+
     private void releaseParameterValues() {
         functionScope = new SymbolTable(functionScope.getParent());
     }
diff --git a/src/function/UserDefinedSpecialFunction.java b/src/function/UserDefinedSpecialFunction.java
new file mode 100644
index 0000000..ea27455
--- /dev/null
+++ b/src/function/UserDefinedSpecialFunction.java
@@ -0,0 +1,16 @@
+package function;
+
+import sexpression.Cons;
+
+public class UserDefinedSpecialFunction extends UserDefinedFunction {
+
+    public UserDefinedSpecialFunction(String name, Cons lambdaList, Cons body) {
+        super(name, lambdaList, body);
+    }
+
+    @Override
+    public boolean evaluateArguments() {
+        return false;
+    }
+
+}
diff --git a/src/function/builtin/EVAL.java b/src/function/builtin/EVAL.java
index b8aa08b..eccbf4b 100644
--- a/src/function/builtin/EVAL.java
+++ b/src/function/builtin/EVAL.java
@@ -21,7 +21,7 @@ public class EVAL extends LispFunction {
     }
 
     private static LispFunction createLambdaFunction(SExpression lambdaExpression) {
-        if (lambdaExpression.functionp())
+        if (lambdaExpression.isFunction())
             return ((LambdaExpression) lambdaExpression).getFunction();
         else if (LAMBDA.isLambdaExpression(lambdaExpression))
             return LAMBDA.createFunction((Cons) lambdaExpression);
@@ -59,14 +59,14 @@ public class EVAL extends LispFunction {
 
         SExpression argument = argumentList.getFirst();
 
-        if (argument.listp()) {
-            if (argument.consp())
+        if (argument.isList()) {
+            if (argument.isCons())
                 return evaluateList((Cons) argument);
 
             return argument; // NIL
         }
 
-        if (argument.symbolp()) {
+        if (argument.isSymbol()) {
             SExpression symbolValue = lookupSymbol(argument.toString());
 
             if (symbolValue != null)
@@ -95,7 +95,7 @@ public class EVAL extends LispFunction {
     }
 
     private Cons evaluateArgList(Cons arguments) {
-        if (arguments.nullp())
+        if (arguments.isNull())
             return Nil.getInstance();
 
         SExpression first = eval(arguments.getFirst());
diff --git a/src/function/builtin/cons/LENGTH.java b/src/function/builtin/cons/LENGTH.java
index 33bb3ef..5b242d7 100644
--- a/src/function/builtin/cons/LENGTH.java
+++ b/src/function/builtin/cons/LENGTH.java
@@ -36,7 +36,7 @@ public class LENGTH extends LispFunction {
         Cons list = (Cons) argumentList.getFirst();
         Cons restOfList = LIST.makeList(list.getRest());
 
-        if (list.nullp())
+        if (list.isNull())
             return new LispNumber(accumulatedLength);
 
         return callTailRecursive(increment(accumulatedLength), restOfList);
diff --git a/src/function/builtin/cons/LIST.java b/src/function/builtin/cons/LIST.java
index cb7a1b6..839e462 100644
--- a/src/function/builtin/cons/LIST.java
+++ b/src/function/builtin/cons/LIST.java
@@ -22,7 +22,7 @@ public class LIST extends LispFunction {
     }
 
     private Cons callRecursive(Cons argumentList) {
-        if (argumentList.nullp())
+        if (argumentList.isNull())
             return Nil.getInstance();
 
         SExpression firstArgument = argumentList.getFirst();
diff --git a/src/function/builtin/math/MathFunction.java b/src/function/builtin/math/MathFunction.java
index 780e5e5..516ad42 100644
--- a/src/function/builtin/math/MathFunction.java
+++ b/src/function/builtin/math/MathFunction.java
@@ -20,7 +20,7 @@ class MathFunction {
         SExpression firstArgument = argumentList.getFirst();
         LispNumber number1 = (LispNumber) firstArgument;
 
-        if (remainingArguments.nullp())
+        if (remainingArguments.isNull())
             return singleValueOperation.apply(number1);
 
         SExpression secondArgument = remainingArguments.getFirst();
@@ -28,7 +28,7 @@ class MathFunction {
         LispNumber operationResult = multipleValueOperation.apply(number1, number2);
         SExpression remainingNumbers = remainingArguments.getRest();
 
-        if (remainingNumbers.nullp())
+        if (remainingNumbers.isNull())
             return operationResult;
 
         return callTailRecursive(new Cons(operationResult, remainingNumbers));
diff --git a/src/function/builtin/predicate/ATOM.java b/src/function/builtin/predicate/ATOM.java
index e645ca7..29c931c 100644
--- a/src/function/builtin/predicate/ATOM.java
+++ b/src/function/builtin/predicate/ATOM.java
@@ -16,7 +16,7 @@ public class ATOM extends LispFunction {
         argumentValidator.validate(argumentList);
         SExpression argument = argumentList.getFirst();
 
-        return argument.atomp() ? Symbol.T : Nil.getInstance();
+        return argument.isAtom() ? Symbol.T : Nil.getInstance();
     }
 
 }
diff --git a/src/function/builtin/predicate/EQ.java b/src/function/builtin/predicate/EQ.java
index 7e6604b..d18c384 100644
--- a/src/function/builtin/predicate/EQ.java
+++ b/src/function/builtin/predicate/EQ.java
@@ -30,7 +30,7 @@ public class EQ extends LispFunction {
     }
 
     private boolean isAtomPair(SExpression firstArgument, SExpression secondArgument) {
-        return firstArgument.atomp() && secondArgument.atomp();
+        return firstArgument.isAtom() && secondArgument.isAtom();
     }
 
     private SExpression atomEq(SExpression firstArgument, SExpression secondArgument) {
diff --git a/src/function/builtin/predicate/EQUAL.java b/src/function/builtin/predicate/EQUAL.java
index f3503a2..6dea5c7 100644
--- a/src/function/builtin/predicate/EQUAL.java
+++ b/src/function/builtin/predicate/EQUAL.java
@@ -41,7 +41,7 @@ public class EQUAL extends LispFunction {
     }
 
     private boolean isListPair(SExpression firstArgument, SExpression secondArgument) {
-        return firstArgument.consp() && secondArgument.consp();
+        return firstArgument.isCons() && secondArgument.isCons();
     }
 
     private SExpression equal(SExpression firstArgument, SExpression secondArgument) {
diff --git a/src/function/builtin/predicate/EQUALSP.java b/src/function/builtin/predicate/EQUALSP.java
index 7a68535..2ab3912 100644
--- a/src/function/builtin/predicate/EQUALSP.java
+++ b/src/function/builtin/predicate/EQUALSP.java
@@ -22,7 +22,7 @@ public class EQUALSP extends LispFunction {
     private SExpression callTailRecursive(Cons argumentList) {
         Cons remainingArguments = (Cons) argumentList.getRest();
 
-        if (remainingArguments.nullp())
+        if (remainingArguments.isNull())
             return Symbol.T;
 
         SExpression firstArgument = argumentList.getFirst();
diff --git a/src/function/builtin/predicate/GREATERP.java b/src/function/builtin/predicate/GREATERP.java
index 15365b5..357363e 100644
--- a/src/function/builtin/predicate/GREATERP.java
+++ b/src/function/builtin/predicate/GREATERP.java
@@ -22,7 +22,7 @@ public class GREATERP extends LispFunction {
     private SExpression callTailRecursive(Cons argumentList) {
         Cons remainingArguments = (Cons) argumentList.getRest();
 
-        if (remainingArguments.nullp())
+        if (remainingArguments.isNull())
             return Symbol.T;
 
         SExpression firstArgument = argumentList.getFirst();
diff --git a/src/function/builtin/predicate/LESSP.java b/src/function/builtin/predicate/LESSP.java
index c8dcd1f..089b461 100644
--- a/src/function/builtin/predicate/LESSP.java
+++ b/src/function/builtin/predicate/LESSP.java
@@ -22,7 +22,7 @@ public class LESSP extends LispFunction {
     private SExpression callTailRecursive(Cons argumentList) {
         Cons remainingArguments = (Cons) argumentList.getRest();
 
-        if (remainingArguments.nullp())
+        if (remainingArguments.isNull())
             return Symbol.T;
 
         SExpression firstArgument = argumentList.getFirst();
diff --git a/src/function/builtin/predicate/LISTP.java b/src/function/builtin/predicate/LISTP.java
index 1418cff..ac8eefa 100644
--- a/src/function/builtin/predicate/LISTP.java
+++ b/src/function/builtin/predicate/LISTP.java
@@ -15,7 +15,7 @@ public class LISTP extends LispFunction {
     public SExpression call(Cons argumentList) {
         argumentValidator.validate(argumentList);
 
-        return argumentList.getFirst().listp() ? Symbol.T : Nil.getInstance();
+        return argumentList.getFirst().isList() ? Symbol.T : Nil.getInstance();
     }
 
 }
diff --git a/src/function/builtin/predicate/NULL.java b/src/function/builtin/predicate/NULL.java
index 7f874a7..fab770b 100644
--- a/src/function/builtin/predicate/NULL.java
+++ b/src/function/builtin/predicate/NULL.java
@@ -15,7 +15,7 @@ public class NULL extends LispFunction {
     public SExpression call(Cons argumentList) {
         argumentValidator.validate(argumentList);
 
-        return argumentList.getFirst().nullp() ? Symbol.T : Nil.getInstance();
+        return argumentList.getFirst().isNull() ? Symbol.T : Nil.getInstance();
     }
 
 }
diff --git a/src/function/builtin/special/AND.java b/src/function/builtin/special/AND.java
index a846b61..6c2abe3 100644
--- a/src/function/builtin/special/AND.java
+++ b/src/function/builtin/special/AND.java
@@ -5,7 +5,7 @@ import static function.builtin.EVAL.eval;
 import function.*;
 import sexpression.*;
 
-public class AND extends LispFunction {
+public class AND extends LispSpecialFunction {
 
     private ArgumentValidator argumentValidator;
 
@@ -23,17 +23,13 @@ public class AND extends LispFunction {
         SExpression currentValue = eval(argumentList.getFirst());
         Cons remainingValues = (Cons) argumentList.getRest();
 
-        if (argumentList.nullp())
+        if (argumentList.isNull())
             return lastValue;
 
-        if (currentValue.nullp())
+        if (currentValue.isNull())
             return currentValue;
 
         return callTailRecursive(remainingValues, currentValue);
     }
 
-    public boolean evaluateArguments() {
-        return false;
-    }
-
 }
diff --git a/src/function/builtin/special/COND.java b/src/function/builtin/special/COND.java
index 87bdeee..e0e1d78 100644
--- a/src/function/builtin/special/COND.java
+++ b/src/function/builtin/special/COND.java
@@ -5,7 +5,7 @@ import static function.builtin.EVAL.eval;
 import function.*;
 import sexpression.*;
 
-public class COND extends LispFunction {
+public class COND extends LispSpecialFunction {
 
     private ArgumentValidator argumentValidator;
 
@@ -22,7 +22,7 @@ public class COND extends LispFunction {
     }
 
     private SExpression callTailRecursive(Cons argumentList) {
-        if (argumentList.nullp())
+        if (argumentList.isNull())
             return Nil.getInstance();
 
         Cons clause = (Cons) argumentList.getFirst();
@@ -42,7 +42,7 @@ public class COND extends LispFunction {
     private SExpression evaluateResult(Cons clause, SExpression test) {
         SExpression lastResultValue = test;
 
-        for (SExpression result = clause.getRest(); result.consp(); result = advanceCons(result))
+        for (SExpression result = clause.getRest(); result.isCons(); result = advanceCons(result))
             lastResultValue = eval(getFirst(result));
 
         return lastResultValue;
@@ -56,8 +56,4 @@ public class COND extends LispFunction {
         return ((Cons) knownCons).getFirst();
     }
 
-    public boolean evaluateArguments() {
-        return false;
-    }
-
 }
diff --git a/src/function/builtin/special/DEFUN.java b/src/function/builtin/special/DEFUN.java
index 8ee5df4..5962c56 100644
--- a/src/function/builtin/special/DEFUN.java
+++ b/src/function/builtin/special/DEFUN.java
@@ -9,7 +9,7 @@ import function.builtin.cons.LIST;
 import sexpression.*;
 import table.FunctionTable;
 
-public class DEFUN extends LispFunction {
+public class DEFUN extends LispSpecialFunction {
 
     private ArgumentValidator argumentValidator;
     private ArgumentValidator lambdaListIsListValidator;
@@ -52,10 +52,6 @@ public class DEFUN extends LispFunction {
         return functionName;
     }
 
-    public boolean evaluateArguments() {
-        return false;
-    }
-
     public class RedefiningFunctionWarning extends LispWarning {
 
         private static final long serialVersionUID = 1L;
diff --git a/src/function/builtin/special/IF.java b/src/function/builtin/special/IF.java
index 2e0c607..79a0f89 100644
--- a/src/function/builtin/special/IF.java
+++ b/src/function/builtin/special/IF.java
@@ -5,7 +5,7 @@ import static function.builtin.EVAL.eval;
 import function.*;
 import sexpression.*;
 
-public class IF extends LispFunction {
+public class IF extends LispSpecialFunction {
 
     private ArgumentValidator argumentValidator;
 
@@ -26,7 +26,7 @@ public class IF extends LispFunction {
     }
 
     private boolean isNil(SExpression test) {
-        return test.nullp();
+        return test.isNull();
     }
 
     private SExpression getThenForm(Cons argumentList) {
@@ -45,8 +45,4 @@ public class IF extends LispFunction {
         return getRestOfList(expressions).getFirst();
     }
 
-    public boolean evaluateArguments() {
-        return false;
-    }
-
 }
diff --git a/src/function/builtin/special/LAMBDA.java b/src/function/builtin/special/LAMBDA.java
index d64d911..84d6e6d 100644
--- a/src/function/builtin/special/LAMBDA.java
+++ b/src/function/builtin/special/LAMBDA.java
@@ -5,10 +5,10 @@ import static function.builtin.cons.LIST.makeList;
 import function.*;
 import sexpression.*;
 
-public class LAMBDA extends LispFunction {
+public class LAMBDA extends LispSpecialFunction {
 
     public static boolean isLambdaExpression(SExpression sexpr) {
-        if (sexpr.consp()) {
+        if (sexpr.isCons()) {
             SExpression first = ((Cons) sexpr).getFirst();
 
             return "LAMBDA".equals(first.toString());
@@ -59,8 +59,4 @@ public class LAMBDA extends LispFunction {
         return new Cons(new Symbol("LAMBDA"), argumentList);
     }
 
-    public boolean evaluateArguments() {
-        return false;
-    }
-
 }
diff --git a/src/function/builtin/special/LET.java b/src/function/builtin/special/LET.java
index 7ac85fe..ce0df32 100644
--- a/src/function/builtin/special/LET.java
+++ b/src/function/builtin/special/LET.java
@@ -6,7 +6,7 @@ import function.*;
 import sexpression.*;
 import table.*;
 
-public class LET extends LispFunction {
+public class LET extends LispSpecialFunction {
 
     private ArgumentValidator argumentValidator;
     private ArgumentValidator variableDefinitionListValidator;
@@ -55,7 +55,7 @@ public class LET extends LispFunction {
     private void addVariablesToScope(SymbolTable scope, Cons variableDefinitions) {
         variableDefinitionListValidator.validate(variableDefinitions);
 
-        for (; variableDefinitions.consp(); variableDefinitions = (Cons) variableDefinitions.getRest())
+        for (; variableDefinitions.isCons(); variableDefinitions = (Cons) variableDefinitions.getRest())
             addPairToScope((Cons) variableDefinitions.getFirst(), scope);
     }
 
@@ -72,7 +72,7 @@ public class LET extends LispFunction {
     private SExpression evaluateBody(Cons body) {
         SExpression lastEvaluation = Nil.getInstance();
 
-        for (; body.consp(); body = (Cons) body.getRest())
+        for (; body.isCons(); body = (Cons) body.getRest())
             lastEvaluation = eval(body.getFirst());
 
         return lastEvaluation;
@@ -82,8 +82,4 @@ public class LET extends LispFunction {
         executionContext.setScope(localScope.getParent());
     }
 
-    public boolean evaluateArguments() {
-        return false;
-    }
-
 }
diff --git a/src/function/builtin/special/OR.java b/src/function/builtin/special/OR.java
index a5e2ce4..7d2d2eb 100644
--- a/src/function/builtin/special/OR.java
+++ b/src/function/builtin/special/OR.java
@@ -5,7 +5,7 @@ import static function.builtin.EVAL.eval;
 import function.*;
 import sexpression.*;
 
-public class OR extends LispFunction {
+public class OR extends LispSpecialFunction {
 
     private ArgumentValidator argumentValidator;
 
@@ -23,14 +23,10 @@ public class OR extends LispFunction {
         SExpression currentValue = eval(argumentList.getFirst());
         Cons remainingValues = (Cons) argumentList.getRest();
 
-        if (remainingValues.nullp() || !currentValue.nullp())
+        if (remainingValues.isNull() || !currentValue.isNull())
             return currentValue;
 
         return callTailRecursive(remainingValues);
     }
 
-    public boolean evaluateArguments() {
-        return false;
-    }
-
 }
diff --git a/src/function/builtin/special/QUOTE.java b/src/function/builtin/special/QUOTE.java
index d92c07d..345218a 100644
--- a/src/function/builtin/special/QUOTE.java
+++ b/src/function/builtin/special/QUOTE.java
@@ -3,7 +3,7 @@ package function.builtin.special;
 import function.*;
 import sexpression.*;
 
-public class QUOTE extends LispFunction {
+public class QUOTE extends LispSpecialFunction {
 
     private ArgumentValidator argumentValidator;
 
@@ -18,8 +18,4 @@ public class QUOTE extends LispFunction {
         return argumentList.getFirst();
     }
 
-    public boolean evaluateArguments() {
-        return false;
-    }
-
 }
diff --git a/src/function/builtin/special/SETF.java b/src/function/builtin/special/SETF.java
index 224962b..70bd318 100644
--- a/src/function/builtin/special/SETF.java
+++ b/src/function/builtin/special/SETF.java
@@ -6,7 +6,7 @@ import function.*;
 import sexpression.*;
 import table.*;
 
-public class SETF extends LispFunction {
+public class SETF extends LispSpecialFunction {
 
     private ArgumentValidator argumentValidator;
     private ExecutionContext executionContext;
@@ -48,8 +48,4 @@ public class SETF extends LispFunction {
         return table.getParent() == null;
     }
 
-    public boolean evaluateArguments() {
-        return false;
-    }
-
 }
diff --git a/src/sexpression/Atom.java b/src/sexpression/Atom.java
index 8e969e7..e007c15 100644
--- a/src/sexpression/Atom.java
+++ b/src/sexpression/Atom.java
@@ -9,7 +9,7 @@ public abstract class Atom extends SExpression {
         this.text = text;
     }
 
-    public boolean atomp() {
+    public boolean isAtom() {
         return true;
     }
 
diff --git a/src/sexpression/Cons.java b/src/sexpression/Cons.java
index 0738bd0..9c26e23 100644
--- a/src/sexpression/Cons.java
+++ b/src/sexpression/Cons.java
@@ -27,7 +27,7 @@ public class Cons extends SExpression {
         this.rest = rest;
     }
 
-    public boolean consp() {
+    public boolean isCons() {
         return true;
     }
 
@@ -36,9 +36,9 @@ public class Cons extends SExpression {
     }
 
     private String toStringAux() {
-        if (rest.nullp())
+        if (rest.isNull())
             return (first.toString() + ")");
-        else if (rest.consp())
+        else if (rest.isCons())
             return (first.toString() + " " + ((Cons) rest).toStringAux());
 
         return (first.toString() + " . " + rest.toString() + ")");
diff --git a/src/sexpression/LambdaExpression.java b/src/sexpression/LambdaExpression.java
index 63a20fb..916fa09 100644
--- a/src/sexpression/LambdaExpression.java
+++ b/src/sexpression/LambdaExpression.java
@@ -13,7 +13,7 @@ public class LambdaExpression extends SExpression {
         this.function = function;
     }
 
-    public boolean functionp() {
+    public boolean isFunction() {
         return true;
     }
 
diff --git a/src/sexpression/LispNumber.java b/src/sexpression/LispNumber.java
index dd90034..d465de6 100644
--- a/src/sexpression/LispNumber.java
+++ b/src/sexpression/LispNumber.java
@@ -29,7 +29,7 @@ public class LispNumber extends Atom {
         this.value = value;
     }
 
-    public boolean numberp() {
+    public boolean isNumber() {
         return true;
     }
 
diff --git a/src/sexpression/LispString.java b/src/sexpression/LispString.java
index db5524f..bc83c81 100644
--- a/src/sexpression/LispString.java
+++ b/src/sexpression/LispString.java
@@ -7,7 +7,7 @@ public class LispString extends Atom {
         super(text);
     }
 
-    public boolean stringp() {
+    public boolean isString() {
         return true;
     }
 
diff --git a/src/sexpression/Nil.java b/src/sexpression/Nil.java
index 0fe3c44..2b54191 100644
--- a/src/sexpression/Nil.java
+++ b/src/sexpression/Nil.java
@@ -16,19 +16,19 @@ public class Nil extends Cons {
         super.setRest(this);
     }
 
-    public boolean nullp() {
+    public boolean isNull() {
         return true;
     }
 
-    public boolean atomp() {
+    public boolean isAtom() {
         return true;
     }
 
-    public boolean consp() {
+    public boolean isCons() {
         return false;
     }
 
-    public boolean symbolp() {
+    public boolean isSymbol() {
         return true;
     }
 
diff --git a/src/sexpression/SExpression.java b/src/sexpression/SExpression.java
index 548a412..cc7d8c4 100644
--- a/src/sexpression/SExpression.java
+++ b/src/sexpression/SExpression.java
@@ -3,35 +3,35 @@ package sexpression;
 @DisplayName("s-expression")
 public abstract class SExpression {
 
-    public boolean nullp() {
+    public boolean isNull() {
         return false;
     }
 
-    public boolean atomp() {
+    public boolean isAtom() {
         return false;
     }
 
-    public boolean consp() {
+    public boolean isCons() {
         return false;
     }
 
-    public boolean listp() {
-        return (consp() || nullp());
+    public boolean isList() {
+        return (isCons() || isNull());
     }
 
-    public boolean numberp() {
+    public boolean isNumber() {
         return false;
     }
 
-    public boolean symbolp() {
+    public boolean isSymbol() {
         return false;
     }
 
-    public boolean functionp() {
+    public boolean isFunction() {
         return false;
     }
 
-    public boolean stringp() {
+    public boolean isString() {
         return false;
     }
 
diff --git a/src/sexpression/Symbol.java b/src/sexpression/Symbol.java
index 289589a..550acdd 100644
--- a/src/sexpression/Symbol.java
+++ b/src/sexpression/Symbol.java
@@ -13,7 +13,7 @@ public class Symbol extends Atom {
         super(text.toUpperCase());
     }
 
-    public boolean symbolp() {
+    public boolean isSymbol() {
         return true;
     }
 
diff --git a/src/token/Token.java b/src/token/Token.java
index fe8af36..7cdf43b 100644
--- a/src/token/Token.java
+++ b/src/token/Token.java
@@ -5,9 +5,6 @@ import java.util.function.Supplier;
 import file.FilePosition;
 import sexpression.*;
 
-/**
- * A token in Lisp.
- */
 public abstract class Token {
 
     private String text;
@@ -26,10 +23,12 @@ public abstract class Token {
         return position;
     }
 
-    // sExpr ::= NUMBER | IDENTIFIER | STRING | QUOTE_MARK sExpr | LEFT_PAREN sExprTail
+    // s-expression ::= NUMBER | IDENTIFIER | STRING | QUOTE_MARK s-expression | LEFT_PAREN
+    // s-expression-tail
+    // s-expression-tail ::= RIGHT_PAREN | s-expression s-expression-tail
+
     public abstract SExpression parseSExpression(Supplier<Token> getNextToken);
 
-    // sExprTail ::= RIGHT_PAREN | sExpr sExprTail
     public SExpression parseSExpressionTail(Supplier<Token> getNextToken) {
         SExpression first = parseSExpression(getNextToken);
 
diff --git a/test/testutil/TypeAssertions.java b/test/testutil/TypeAssertions.java
index 278aede..a00a756 100644
--- a/test/testutil/TypeAssertions.java
+++ b/test/testutil/TypeAssertions.java
@@ -7,61 +7,61 @@ import sexpression.*;
 public final class TypeAssertions {
 
     public static void assertList(SExpression sExpression) {
-        assertFalse(sExpression.atomp());
-        assertTrue(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertTrue(sExpression.listp());
-        assertFalse(sExpression.nullp());
-        assertFalse(sExpression.numberp());
-        assertFalse(sExpression.stringp());
-        assertFalse(sExpression.symbolp());
+        assertFalse(sExpression.isAtom());
+        assertTrue(sExpression.isCons());
+        assertFalse(sExpression.isFunction());
+        assertTrue(sExpression.isList());
+        assertFalse(sExpression.isNull());
+        assertFalse(sExpression.isNumber());
+        assertFalse(sExpression.isString());
+        assertFalse(sExpression.isSymbol());
     }
 
     public static void assertNil(SExpression sExpression) {
         assertEquals(Nil.getInstance(), sExpression);
 
-        assertTrue(sExpression.atomp());
-        assertFalse(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertTrue(sExpression.listp());
-        assertTrue(sExpression.nullp());
-        assertFalse(sExpression.numberp());
-        assertFalse(sExpression.stringp());
-        assertTrue(sExpression.symbolp());
+        assertTrue(sExpression.isAtom());
+        assertFalse(sExpression.isCons());
+        assertFalse(sExpression.isFunction());
+        assertTrue(sExpression.isList());
+        assertTrue(sExpression.isNull());
+        assertFalse(sExpression.isNumber());
+        assertFalse(sExpression.isString());
+        assertTrue(sExpression.isSymbol());
 
     }
 
     public static void assertNumber(SExpression sExpression) {
-        assertTrue(sExpression.atomp());
-        assertFalse(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertFalse(sExpression.listp());
-        assertFalse(sExpression.nullp());
-        assertTrue(sExpression.numberp());
-        assertFalse(sExpression.stringp());
-        assertFalse(sExpression.symbolp());
+        assertTrue(sExpression.isAtom());
+        assertFalse(sExpression.isCons());
+        assertFalse(sExpression.isFunction());
+        assertFalse(sExpression.isList());
+        assertFalse(sExpression.isNull());
+        assertTrue(sExpression.isNumber());
+        assertFalse(sExpression.isString());
+        assertFalse(sExpression.isSymbol());
     }
 
     public static void assertString(SExpression sExpression) {
-        assertTrue(sExpression.atomp());
-        assertFalse(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertFalse(sExpression.listp());
-        assertFalse(sExpression.nullp());
-        assertFalse(sExpression.numberp());
-        assertTrue(sExpression.stringp());
-        assertFalse(sExpression.symbolp());
+        assertTrue(sExpression.isAtom());
+        assertFalse(sExpression.isCons());
+        assertFalse(sExpression.isFunction());
+        assertFalse(sExpression.isList());
+        assertFalse(sExpression.isNull());
+        assertFalse(sExpression.isNumber());
+        assertTrue(sExpression.isString());
+        assertFalse(sExpression.isSymbol());
     }
 
     public static void assertSymbol(SExpression sExpression) {
-        assertTrue(sExpression.atomp());
-        assertFalse(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertFalse(sExpression.listp());
-        assertFalse(sExpression.nullp());
-        assertFalse(sExpression.numberp());
-        assertFalse(sExpression.stringp());
-        assertTrue(sExpression.symbolp());
+        assertTrue(sExpression.isAtom());
+        assertFalse(sExpression.isCons());
+        assertFalse(sExpression.isFunction());
+        assertFalse(sExpression.isList());
+        assertFalse(sExpression.isNull());
+        assertFalse(sExpression.isNumber());
+        assertFalse(sExpression.isString());
+        assertTrue(sExpression.isSymbol());
     }
 
     public static void assertT(SExpression sExpression) {