[eclipse-clp-users] X is p(a) +1. from Apt/Wallace book

In the Apt/Wallace book Constraint Log Programming Using Eclipse (2007),
Ch 3 "Arithmetic in Prolog" on pages 45-46 there is this code:
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To save explicitly introducing such extra variables, ECLiPSe supports 
for is/2 a functional syntax. Let us suppose p/2 is a predicate whose 
last argument is numeric, viz.

p(a, 1).
p(b, 2).

Then p/1 can be used as a function when it occurs on the right-hand side
of is, for example:

[eclipse 10]: X is p(a)+1.

X = 2
Yes (0.00s cpu)

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How does this work?  I don't understand how p(a,1) becomes numeric.

Can you point me to some reference to "predicate whose last
argument is numeric" in the manuals?

Thanks.

-DAE