anchor.diff:  /// 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 {
  +class LetPrim extends Expression implements InteriorNode {
     final Primitive primitive;
     Expression body = null;
   
  
positive.code: /// Continuations are normally bound by 'let cont'.  A continuation with no
  /// parameter (or body) is used to represent a function's return continuation.
  /// The return continuation is bound by the Function, not by 'let cont'.
  class Continuation extends Definition {
    final List<Parameter> parameters;
    Expression body = null;
  
  
negative.code: ///
  ///   let rec [variable] = [definition] in [body]
  ///
  class DeclareFunction extends Expression implements InteriorNode {
    final Local variable;
    final FunctionDefinition definition;
    Expression body;
  