Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
demptĭo, ōnis, f. [demo], a taking away (very rare): demptio aut additio literarum, Varr. L. L. 5, § 6 Müll.: damnum a demptione, id. ib. 5, § 176.