anchor.diff:  import 'scope.dart';
   import 'turtle.dart';
   
  -/**
  - * Something went wrong. TODO: useful messaging
  - */
  +/// Something went wrong. TODO: useful messaging
   class InterpreterException {
     final String message;
     const InterpreterException(this.message);
   }
   
  -/**
  - * Used for OUTPUT and STOP.
  - */
  +/// Used for OUTPUT and STOP.
   class InterpreterOutputException {
     final Node result;
     const InterpreterOutputException(this.result);
  
positive.code:     return (100 * src).round() / 100;
    }
  
    /**
     * Evaluates a primitive function (aka command/operator).
     * 
     * Evaluates the primitive function [p] with (yet unevaluated) arguments
     * available in [nodes]. Returns the (uninterpreted) tail.
     */
    ListNode evalPrimFun(Primitive p, ListNode nodes, Scope scope) {
      if (p == Primitive.QUOTE) {
        return nodes;
  
negative.code:     return result;
    }
  
    /// Interprets user-defined function (aka command/operator).
    ///
    /// Takes a definition node [defn] and [scope].
    ListNode applyUserFun(DefnNode defn, ListNode tail, Scope scope) {
      ListNode formalParams = defn.vars;
      final body = defn.body;
  