better handling of comments

ml.h

@@ -16,7 +16,7 @@ class MlValue;
 class MlEnvironment {
 public:
     // Default constructor
-    MlEnvironment() : parent_scope(NULL) {}
+    MlEnvironment() : parent_scope(nullptr) {}
 
     // Does this environment, or its parent environment,
     // have this atom in scope?
@@ -120,7 +120,7 @@ public:
 
     bool is_number() const;
 
-    // Get the "truthy" boolean value of this value.
+    // Get the boolean value of this value.
     bool as_bool() const;
 
     // Get this item's integer value