anchor.diff:  
      Formatting specifiers take the form `{...}`, where `...` includes
      both a _reference_ (what's to be output) and a _formatter_ (how to
  -   output it).
  +                                                                   output it).
   
      - If the reference starts with `#`, it is an implicit named symbol
  -     (something in scope, and not passed explicitly).
  +   (something in scope, and not passed explicitly).
      - If the reference starts with an alphabetic character, it is
  -     _named_: something given to the [[format]] macro explicitly, as a
  -     keyword argument.
  +   _named_: something given to the [[format]] macro explicitly, as a
  +   keyword argument.
      - If the reference starts with `$`, it is a positional argument.
   
      The formatter can either start with `:`, in which case it references
  
positive.code: (import base (defun defmacro > = when n list tonumber error /= + or ..
                else - >= values-list print unpack if apply const-val
                type# and get-idx gensym))
  (import match (case function))
  (import list (cons dolist push-cdr! nth car cdr maybe-map map))
  (import table (.> .<!))
  
negative.code:                                [((_ (positional ?d) _)) d]
                                 [else nil])
                               fragments)))
           (named-map (gensym))
           ((positionals nameds)
            (loop [(pos '())
                   (nam {}) (expecting-nam nil)
  