Can be obtained here As of 2012-09-19, "www.mapcar.org" could not be resolved by DNS.
Still in an early stage, with only a few functions implemented.
See home page
Some internal details:
Uses `tries' to hash the given substitutions. Operates on streams using methods that operate on a class called backtracking-buffer the source for which is currently included along with the rest (though it is completely separable).