anchor.diff:                  (mapcar (lambda (rhs-expr)
                             (list '= lhs-expr rhs-expr))
                           ;; remove all but the expressions whose value is the same as lhs-expr
  -                        (remove lhs-expr (make-exprs rhs)
  +                        (remove (eval lhs-expr) (make-exprs rhs)
                             :test-not #'eql
                             :key #'eval)))
                 (make-exprs lhs))
  
positive.code: 
  (defparameter *unary-ops*  '(-) "Unary operators.")
  (defparameter *binary-ops* '(= + - * /) "Binary operators, sorted by precedence.")
  
  (defun prefix-to-infix (expr)
    "Convert a Lisp arithmetic expression in Polish notation into a list of numbers and operators in infix notation.
  
negative.code:                 (mapcar (lambda (lhs-expr)
                            (list '= lhs-expr rhs-expr))
                          ;; remove all but the expressions whose value is the same as rhs-expr
                          (remove (eval rhs-expr) (make-exprs lhs)
                            :test-not #'eql
                            :key #'eval)))
                (make-exprs rhs))))
  