This program uses a table of pronounceable letter-triples derived from a
dictionary scan. Each potential mnemonic must be such that all of its
letter-triples are in the table to be emitted. About 30% of possible triples
are considered pronounceable. A typical 7-digit phone number has 19,683
possible mnemonics, but this test usually cuts the list down to a few hundred
or so, a reasonable number to eyeball-check. For some numbers, the list will,
sadly, be empty.