In a sum, x+y, we call x and y "summands"

This type of word (from a Latin participle ending) occurs elsewhere in
mathematics. A "summand" is that which is to be summed.

In a composition of functions [Tex $f \circ g$ ], what word should be
used for the two items f, g? How about "composand" ?

Google search shows John Baez used "composand" (in the addendum to
his Week 220), as well as a few other mathematicians. [Not counting
lots of non-math French-language hits, not what we want.]

What do you think? Is there another name (less general
than "operand") in use for this?

G. A. Edgar