cems property on a document or any document element to get a list of the chemical entity mentions:
>>> doc.cems [Span('5,10,15,20-Tetra(4-carboxyphenyl)porphyrin', 19, 61), Span('THF', 82, 85), Span('Tetrahydrofuran', 65, 80)]
Each mention is returned as a
Span, which contains the mention text, as well as the start and end character
offsets within the containing document element.