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The Collaborative International Dictionary of English v.0.48 (gcide)

Com- \Com-\
A prefix from the Latin preposition cum, signifying with,
together, in conjunction, very, etc. It is used in the form
com- before b, m, p, and sometimes f, and by assimilation
becomes col- before l, cor- before r, and con- before any
consonant except b, h, l, m, p, r, and w. Before a vowel com-
becomes co-; also before h, w, and sometimes before other
consonants.
[1913 Webster]