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: (defun make-exprs (numbers)
    "Generate all expressions formed by inserting operators to NUMBERS."
    (if (= (length numbers) 1)
        (apply-unary-ops (car numbers))
        ;; split numbers, combine each expression from either partition using a binary operator
        (loop for i from 1 to (1- (length numbers))
              for partition = (split numbers i) append 
  
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))))
  