I realized the way to do this is to define predecessor using PR.
pred = PR Z (P 1)
returns x-1
or zero if x = 0
.
From there modus can be defined as follows
modus = C modus' [P 2, P 1]
modus' = PR P 1 (C pred [P 2])
Which recursively decrements P 1
P 2
times or until P 1
is equal to zero.