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 implements InteriorNode {
  +class LetPrim extends Expression {
     final Primitive primitive;
     Expression body = null;
   
  
positive.code: ///
  ///   let rec [variable] = [definition] in [body]
  ///
  class DeclareFunction extends Expression implements InteriorNode {
    final Local variable;
    final FunctionDefinition definition;
    Expression body;
  
negative.code: abstract class Invoke {
    Selector get selector;
    List<Reference> get arguments;
  }
  
  /// Invoke a static function or static field getter/setter.
  