anchor.diff:  (defun progn-statements (stmt)
     (match stmt
       (('progn . stmts) stmts)
  -    (_ (error (format nil "invalid statement: ~A" stmt)))))
  +    (_ (error "invalid statement: ~A" stmt))))
   
   (defun compile-progn (stmt type-env def)
     (compile-progn-statements (progn-statements stmt) type-env def))
  
positive.code:                                         (compile-expression exp type-env def))
                                      sizes))
                  (%compile-with-shared-memory rest stmts type-env2 def))))
      (_ (error (format nil "invalid shared memory specs: ~A" specs)))))
  
  (defun compile-with-shared-memory-statements (stmts type-env def)
    (compile-let-statements stmts type-env def))
  
negative.code: (defun set-place (stmt)
    (match stmt
      (('set place _) place)
      (_ (error "invalid statement: ~A" stmt))))
  
  (defun set-expression (stmt)
    (match stmt
      (('set _ exp) exp)
      (_ (error "invalid statement: ~A" stmt))))
  
  (defun compile-set (stmt type-env def)
    (let ((place (set-place stmt))
  