Algorithm that converts numeric amount into English words

Question

What is the most efficient way to convert numeric amount into English words

e.g. 12 to twelve 127 to one hundred twenty-seven

Answer 1

This is part of Common Lisp!

Here's how GNU CLISP does it, and here's how CMUCL does it (easier to read, IMHO).

Doing a code search for "format million billion" will turn up lots of them.