Check whether obj is a corpus. Return (is_corpus, new) 2-tuple, where new is obj if obj was an iterable, or new yields the same sequence as obj if it was an iterator.

obj is a corpus if it supports iteration over documents, where a document is in turn anything that acts as a sequence of 2-tuples (int, float).

Note: An “empty” corpus (empty input sequence) is ambiguous, so in this case the result is forcefully defined as is_corpus=False.