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

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.

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.