def _alphabet_from_rdata(rdata, void_label, dummy_label):
"""Extract alphabet (of observations and labels) from
given raw data."""
alphabet = Alphabet(void_label=void_label, dummy_label=dummy_label)
for (sent, labels) in rdata:
for word in sent:
for x in chain(*word):
alphabet.add_observation(x)
for y in labels:
alphabet.add_label(y)
return alphabet
