anchor.diff:                 list or pretty ))
   (import type ( eq? list? symbol? string?
                  boolean? number? ))
  -(import list ( car caddr cadr cdr
  +(import list ( car caddr cadr cdr append for-each
                  map filter push-cdr!
                  nth ))
   (import string (..))
  
positive.code:       [(list? pattern)
         (cond
           [(cons-pattern? pattern)
            (list (compile-pattern-bindings (car pattern) `(car ,symb))
                  (compile-pattern-bindings (caddr pattern) `(cdr ,symb)))]
           [true
            (let* [(out '())]
              (for i 1 (# pattern) 1
                (push-cdr! out (compile-pattern-bindings (nth pattern i) `(nth ,symb ,i))))
              out)])]
        [(symbol? pattern)
         `(,pattern ,symb)]
        [(or (number? pattern) (boolean? pattern) (string? pattern) (eq? pattern '_))
         '()]
        [true (error (.. "unsupported pattern " (pretty pattern)))])))
  
negative.code:     `(let* [(,val-sym ,val)]
         (cond ,@(map compile-arm pts)
               [true (error "pattern matching failure")]))))
  