The Extractor argument extraction keeps ; together, so we use
that to delimit options. Actually, the only option supported is
mapdigits, which tells the Distsim to try mapping [0-9] to 0 and
requery for an unknown word with digits.
Returns the cluster for the given word as a string. If the word
is not found, but the Distsim contains default numbers and the
word contains the digits 0-9, the default number is returned if
found. If the word is still unknown, the unknown word is
returned ("null" if no other unknown word was specified).