anchor.diff:  abstract class Invoke {
     Selector get selector;
     List<Reference> get arguments;
  -}
  -
  -/// Represents a node with a child node, which can be accessed through the
  -/// `body` member. A typical usage is when removing a node from the CPS graph:
  -///
  -///     Node child          = node.body;
  -///     InteriorNode parent = node.parent;
  -///
  -///     child.parent = parent;
  -///     parent.body  = child;
  -abstract class InteriorNode implements Node {
  -  Expression body;
   }
   
   /// Invoke a static function or static field getter/setter.
  
positive.code: 
  /// A function definition, consisting of parameters and a body.  The parameters
  /// include a distinguished continuation parameter.
  class FunctionDefinition extends Node implements InteriorNode {
    final FunctionElement element;
    final Continuation returnContinuation;
    final List<Parameter> parameters;
    Expression body;
    final List<ConstDeclaration> localConstants;
  
    /// Values for optional parameters.
  
negative.code: /// value is in scope in the body.
  /// During one-pass construction a LetVal with an empty body is used to
  /// represent one-level context 'let val x = V in []'.
  class LetPrim extends Expression {
    final Primitive primitive;
    Expression body = null;
  
  