diff --git a/src/main/kotlin/function/ArgumentValidator.kt b/src/main/kotlin/function/ArgumentValidator.kt
index c22dcce..35cc2ed 100644
--- a/src/main/kotlin/function/ArgumentValidator.kt
+++ b/src/main/kotlin/function/ArgumentValidator.kt
@@ -63,18 +63,12 @@ class ArgumentValidator(private val functionName: String) {
     }
 
     private fun validateListNotDotted(argumentList: Cons) {
-        var current = argumentList
-        var next = current.rest
-
-        while (next.isCons) {
-            current = next as Cons
-            next = current.rest
-        }
-
-        if (!next.isNull)
+        if (isArgumentListDotted(argumentList))
             throw DottedArgumentListException(functionName, argumentList)
     }
 
+    private fun isArgumentListDotted(argumentList: Cons) = argumentList.isCons && !argumentList.last().rest.isNull
+
     private fun validateListLength(argumentList: Cons) {
         if (containsTooFewArguments(argumentList))
             throw TooFewArgumentsException(functionName, argumentList)
@@ -110,13 +104,11 @@ class ArgumentValidator(private val functionName: String) {
         excludedFirstArgumentType != null && excludedFirstArgumentType!!.isInstance(firstArgument)
 
     private fun validateTrailingArguments(argumentList: Cons) {
-        var cons = argumentList.rest as Cons
+        val cons = argumentList.rest as Cons
 
-        while (!cons.isNull) {
-            if (!isExpectedTrailingArgumentType(cons.first))
-                throw BadArgumentTypeException(functionName, cons.first, trailingArgumentType)
-
-            cons = cons.rest as Cons
+        cons.forEach {
+            if (!isExpectedTrailingArgumentType(it.first))
+                throw BadArgumentTypeException(functionName, it.first, trailingArgumentType)
         }
     }
 
diff --git a/src/main/kotlin/sexpression/AtSignExpression.kt b/src/main/kotlin/sexpression/AtSignExpression.kt
index 7f420e9..9ba271e 100644
--- a/src/main/kotlin/sexpression/AtSignExpression.kt
+++ b/src/main/kotlin/sexpression/AtSignExpression.kt
@@ -2,8 +2,7 @@ package sexpression
 
 class AtSignExpression(val expression: SExpression) : SExpression() {
 
-    override val isAtSign: Boolean
-        get() = true
+    override val isAtSign = true
 
     override fun toString() = "@$expression"
 }
diff --git a/src/main/kotlin/sexpression/Atom.kt b/src/main/kotlin/sexpression/Atom.kt
index 2cd3917..b87820a 100644
--- a/src/main/kotlin/sexpression/Atom.kt
+++ b/src/main/kotlin/sexpression/Atom.kt
@@ -3,8 +3,7 @@ package sexpression
 @DisplayName("atom")
 abstract class Atom(private val text: String) : SExpression() {
 
-    override val isAtom: Boolean
-        get() = true
+    override val isAtom = true
 
     override fun toString() = text
 }
diff --git a/src/main/kotlin/sexpression/BackquoteExpression.kt b/src/main/kotlin/sexpression/BackquoteExpression.kt
index 4dbe500..d205f96 100644
--- a/src/main/kotlin/sexpression/BackquoteExpression.kt
+++ b/src/main/kotlin/sexpression/BackquoteExpression.kt
@@ -2,8 +2,7 @@ package sexpression
 
 class BackquoteExpression(val expression: SExpression) : SExpression() {
 
-    override val isBackquote: Boolean
-        get() = true
+    override val isBackquote = true
 
     override fun toString() = "`$expression"
 }
diff --git a/src/main/kotlin/sexpression/CommaExpression.kt b/src/main/kotlin/sexpression/CommaExpression.kt
index ca93423..f35429b 100644
--- a/src/main/kotlin/sexpression/CommaExpression.kt
+++ b/src/main/kotlin/sexpression/CommaExpression.kt
@@ -2,8 +2,7 @@ package sexpression
 
 class CommaExpression(val expression: SExpression) : SExpression() {
 
-    override val isComma: Boolean
-        get() = true
+    override val isComma = true
 
     override fun toString() = ",$expression"
 }
diff --git a/src/main/kotlin/sexpression/Cons.kt b/src/main/kotlin/sexpression/Cons.kt
index d0972ac..a0b06a2 100644
--- a/src/main/kotlin/sexpression/Cons.kt
+++ b/src/main/kotlin/sexpression/Cons.kt
@@ -6,10 +6,9 @@ import recursion.TailCalls.done
 import recursion.TailCalls.tailCall
 
 @DisplayName("list")
-open class Cons(open var first: SExpression, open var rest: SExpression) : SExpression() {
+open class Cons(open var first: SExpression, open var rest: SExpression) : SExpression(), Iterable<Cons> {
 
-    override val isCons: Boolean
-        get() = true
+    override val isCons = true
 
     override fun toString(): String {
         return toStringTailRecursive(StringBuilder("(")).invoke()
@@ -26,4 +25,14 @@ open class Cons(open var first: SExpression, open var rest: SExpression) : SExpr
 
         return done(leadingString.append(" . " + rest.toString() + ")").toString())
     }
+
+    override fun iterator(): Iterator<Cons> = ConsIterator(this)
+
+    private class ConsIterator(start: Cons) : Iterator<Cons> {
+
+        private var current: SExpression = start
+
+        override fun hasNext() = current.isCons
+        override fun next() = (current as Cons).apply { current = rest }
+    }
 }
diff --git a/src/main/kotlin/sexpression/LambdaExpression.kt b/src/main/kotlin/sexpression/LambdaExpression.kt
index 1138fec..2c4fe70 100644
--- a/src/main/kotlin/sexpression/LambdaExpression.kt
+++ b/src/main/kotlin/sexpression/LambdaExpression.kt
@@ -5,8 +5,7 @@ import function.UserDefinedFunction
 @DisplayName("lambda-expression")
 class LambdaExpression(val lambdaExpression: Cons, val function: UserDefinedFunction) : SExpression() {
 
-    override val isFunction: Boolean
-        get() = true
+    override val isFunction = true
 
     override fun toString() = lambdaExpression.toString()
 }
diff --git a/src/main/kotlin/sexpression/LispNumber.kt b/src/main/kotlin/sexpression/LispNumber.kt
index ea4cc1b..b961e2d 100644
--- a/src/main/kotlin/sexpression/LispNumber.kt
+++ b/src/main/kotlin/sexpression/LispNumber.kt
@@ -12,8 +12,7 @@ class LispNumber : Atom {
     var value: BigInteger? = null
         private set
 
-    override val isNumber: Boolean
-        get() = true
+    override val isNumber = true
 
     constructor(text: String) : super(text.replaceFirst("^0+(?!$)".toRegex(), "")) {
         try {
@@ -27,7 +26,7 @@ class LispNumber : Atom {
         this.value = value
     }
 
-    inner class InvalidNumberException(private val text: String) : LispException() {
+    class InvalidNumberException(private val text: String) : LispException() {
         override val message: String
             get() = format("{0} is not a valid integer", text)
     }
diff --git a/src/main/kotlin/sexpression/LispString.kt b/src/main/kotlin/sexpression/LispString.kt
index fc1f8cb..17dd0d1 100644
--- a/src/main/kotlin/sexpression/LispString.kt
+++ b/src/main/kotlin/sexpression/LispString.kt
@@ -3,6 +3,5 @@ package sexpression
 @DisplayName("string")
 class LispString(text: String) : Atom(text) {
 
-    override val isString: Boolean
-        get() = true
+    override val isString = true
 }
diff --git a/src/main/kotlin/sexpression/Nil.kt b/src/main/kotlin/sexpression/Nil.kt
index 705b290..95f91f0 100644
--- a/src/main/kotlin/sexpression/Nil.kt
+++ b/src/main/kotlin/sexpression/Nil.kt
@@ -11,17 +11,10 @@ object Nil : Cons(T, T) {
     override var rest: SExpression = this
         set(_) {}
 
-    override val isNull: Boolean
-        get() = true
-
-    override val isAtom: Boolean
-        get() = true
-
-    override val isCons: Boolean
-        get() = false
-
-    override val isSymbol: Boolean
-        get() = true
+    override val isNull = true
+    override val isAtom = true
+    override val isCons = false
+    override val isSymbol = true
 
     override fun toString() = "NIL"
 }
diff --git a/src/main/kotlin/sexpression/SExpression.kt b/src/main/kotlin/sexpression/SExpression.kt
index dcdf92c..bcc345e 100644
--- a/src/main/kotlin/sexpression/SExpression.kt
+++ b/src/main/kotlin/sexpression/SExpression.kt
@@ -3,7 +3,7 @@ package sexpression
 @DisplayName("s-expression")
 abstract class SExpression {
 
-    open val isNull: Boolean
+    open val isNull
         get() = false
 
     open val isAtom: Boolean
diff --git a/src/main/kotlin/sexpression/Symbol.kt b/src/main/kotlin/sexpression/Symbol.kt
index b5eeca7..d2801b5 100644
--- a/src/main/kotlin/sexpression/Symbol.kt
+++ b/src/main/kotlin/sexpression/Symbol.kt
@@ -5,8 +5,7 @@ import java.util.Locale
 @DisplayName("symbol")
 open class Symbol(text: String) : Atom(text.toUpperCase(Locale.ROOT)) {
 
-    override val isSymbol: Boolean
-        get() = true
+    override val isSymbol = true
 
     companion object {
         val T = Symbol("T")