anchor.diff:  ;;; The second value is a list of "prerequisite" statements that should go before the
   ;;; innermost enclosing non-nested statement (which may be this statement).
   
  -;;================================================================================
  -;;;; Sub-expression protocol
  +;;;; ======= Sub-expression protocol ===============================================================
   ;;;
   ;;; Transformations are responsible for knowing whether they are nested within subexpressions
   ;;; or not, and therefore whether they should return using a proxy.  For example, the element
  
positive.code: ;;;   var x = 10;
  ;;;   var y = 20;
  
  ;;================================================================================
  ;;;; Implementation note
  ;;;
  ;;; The explicitize transformation has a more complicated protocol than most of
  ;;; the other transformations, so the TRANSFORM method is just a thin wrapper that
  
negative.code:   `(let ((*nested-context* t))
      ,@body))
  
  ;;;; ======= TRANSFORM method ======================================================================
  
  (defmethod transform ((xform (eql 'explicitize)) elm)
    (multiple-value-bind (proxy prereqs)
  