-- Lecture 3 : Conditional
-- page 28
mod! BASIC-NAT{
  [Zero NzNat < Nat]
  op 0 : -> Zero
  op s_ : Nat -> NzNat
}
mod! NAT-EVEN{
  pr(BASIC-NAT)
  pred even_ : Nat 
  var N : Nat 
  eq even 0 = true .
  ceq even(s N) = false if even N .
  ceq even(s N) = true  if not (even N) .
}

select NAT-EVEN .
set trace on
red even s 0 .
set trace off

-- page 30
mod! INFINITE{
  [Elt]
  pred f : Elt 
  var X : Elt
  ceq f(X) = true  if f(X) .
}
select INFINITE 
-- red f(X:Elt) .