manual_conversion_impl
======================

A conversion that implements only a lookup table established explcitly by the user.

add
===

	some_mc add src_pattern tgt_pattern

Indicate that in future, apply of `some_mc` should return `tgt_pattern` if the
input is `src_pattern`.

remove
======

	some_mc remove src_pattern

Indicate that in future, apply of `some_mc` will not return the previously stored
value when `src_pattern` is the input; this re-instates the fallback.