% Peano representation of natural numbers:
% z(ero) is a natural number
% if n is a natural number, then also succesor(n), i.e., s(n)

% is the argument a natural number?
isPeano(z).
isPeano(s(N)) :- isPeano(N).

% relate a number to its succesor
succ(N,s(N)).
% relate a number to its predecessor
pred(s(N),N).

% addition as a relation between three natural numbers:
add(z   ,N,N).
add(s(M),N,s(Z)) :- add(M,N,Z).

% subtraction as a relation between three natural numbers:
sub(M,N,Z) :- add(N,Z,M).

% multiplication as a relation between three natural numbers:
mult(z,_,z).
mult(s(M),N,Z) :- mult(M,N,R), add(R,N,Z).

% less-or-equal:
leq(z,_).
leq(s(M),s(N)) :- leq(M,N).