A naive implmentation of Boolean types and nil node has been added

src/main/java/serene/simple/FalseNode.java

@@ -5,7 +5,7 @@ public class FalseNode extends Node {
 
     @Override
     public Object eval(Scope scope) {
-        return false;
+        return Boolean.FALSE;
     }
 
     @Override

src/main/java/serene/simple/IfSpecialForm.java

@@ -17,7 +17,7 @@ public class IfSpecialForm extends SpecialForm {
     @Override
     public Object eval(final Scope scope) {
         Object result = this.pred.eval(scope);
-        if (result == null) {
+        if (result == null || (result instanceof Boolean && (Boolean) result == false)) {
             return this.elseNode.eval(scope);
         }
         return this.ifNode.eval(scope);

src/main/java/serene/simple/Main.java

@@ -43,7 +43,12 @@ public class Main {
 
                 if (result != ListNode.EMPTY) {
                     System.out.print(";; ");
-                    System.out.println(result.toString());
+                    if (result == null) {
+                        System.out.println("nil");
+                    }
+                    else {
+                        System.out.println(result.toString());
+                    }
                 }
             }
             catch(Exception e) {

src/main/java/serene/simple/NilNode.java

@@ -5,7 +5,7 @@ public class NilNode extends Node {
 
     @Override
     public Object eval(Scope scope) {
-        return this;
+        return null;
     }
 
     @Override

src/main/java/serene/simple/Reader.java

@@ -83,7 +83,17 @@ public class Reader {
             str = str + (char) ch;
         }
 
-        return new SymbolNode(str);
+        switch(str) {
+        case "true":
+            return new TrueNode();
+        case "false":
+            return new FalseNode();
+        case "nil":
+            return new NilNode();
+        default:
+            return new SymbolNode(str);
+        }
+
     }
 
     private static Node readNumber(PushbackReader inputStream) throws IOException {