def _is_subspan(self, m, span):
return (<EOL>m.sentence.id == span[<NUM_LIT:0>]<EOL>and m.char_start >= span[<NUM_LIT:1>]<EOL>and m.char_end <= span[<NUM_LIT:2>]<EOL>)<EOL>
Tests if mention m is subspan of span, where span is defined
specific to mention type.