This package implements the macro `##', which provides compact
syntax for short `lambda', without actually being new syntax,
which would be difficult to get merged into Emacs.  Past attempts
to add syntax were met with determined pushback and the use of a
macro was suggested as an alternative.

The `##' macro, whose signature is (## FN &rest args), expands
to a `lambda' expression, which wraps around its arguments.

This `lambda' expression calls the function FN with arguments
ARGS and returns its value.  Its own arguments are derived from
symbols found in ARGS.

Each symbol from `%1' through `%9', which appears in ARGS,
specifies an argument.  Each symbol from `&1' through `&9', which
appears in ARGS, specifies an optional argument.  All arguments
following an optional argument have to be optional as well, thus
their names have to begin with `&'.  Symbol `&*' specifies extra
(`&rest') arguments.

Instead of `%1', the shorthand `%' can be used; but that should
only be done if it is the only argument, and using both `%1' and
`%' is not allowed.  Likewise `&' can be substituted for `&1'.
Finally, for backward compatibility, `%*' can be used in place
of `&*', but only if there are no optional arguments.

Instead of:

  (lambda (a _ &optional c &rest d)
    (foo a (bar c) d))

you can use this macro and write:

  (##foo %1 (bar &3) &*)

which expands to:

  (lambda (%1 _%2 &optional &3 &rest &*)
    (foo %1 (bar &3) &*))

The name `##' was chosen because that allows (optionally)
omitting the whitespace between it and the following symbol.
It also looks similar to #'function.