Kenneth G. Wilson (1923). The Columbia Guide to Standard American English. 1993.
-cede, -ceed, -sede (word-forming roots)
All these are pronounced like seed, but the three different spellings cause problems. Most English words are spelled with -cede, as in accede, concede, precede, recede, and the like. But proceed, exceed, and succeed differ, and supersede is one of a kind.