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Here is a good idea, I think

    Date: 15 October 1982 0010-EDT (Friday)
    From: Guy.Steele at CMU-10A
    Re:   Here is a good idea, I think

    Presumably (EVENP x y)  <=>  (ZEROP (MOD x y)) ?

My mathematical training makes me think it more natural to define:

    (ZEROP x y) <=> (ZEROP (MOD x y))

So that you can think of the second argument to ZEROP as specifing the ring
that you are considering the first argument a member of.  In fact, if it
wasn't about 20 years too late, I might suggest

    (PLUS x y m) <=> (MOD (PLUS x y) m)

and so forth for all the other arithmetic functions.  There are some real
benefits to be gained from knowing the modulus before you start to compute
some functions (EXPT for example).

Or do we already have a meaning for the second argument to ZEROP?