anchor.diff:  ///
   ///   let rec [variable] = [definition] in [body]
   ///
  -class DeclareFunction extends Expression {
  +class DeclareFunction extends Expression implements InteriorNode {
     final Local variable;
     final FunctionDefinition definition;
     Expression body;
  
positive.code: ///
  /// Closure variables without a declaring [SetClosureVariable] are implicitly
  /// declared at the entry to the [variable]'s enclosing function.
  class SetClosureVariable extends Expression {
    final Local variable;
    final Reference value;
    Expression body;
  
negative.code: 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.
  