Editing Gödel (programming language) (section)

==Sample code==

The following Gödel module is a specification of the greatest common divisor (GCD) of two numbers. It is intended to demonstrate the declarative nature of Gödel, not to be particularly efficient.
The <code>CommonDivisor</code> predicate says that if <code>i</code> and <code>j</code> are not zero, then <code>d</code> is a common divisor of <code>i</code> and <code>j</code> if it lies between <code>1</code> and the smaller of <code>i</code> and <code>j</code> and divides both <code>i</code> and <code>j</code> exactly.
The <code>Gcd</code> predicate says that <code>d</code> is a greatest common divisor of <code>i</code> and <code>j</code> if it is a common divisor of <code>i</code> and <code>j</code>, and there is no <code>e</code> that is also a common divisor of <code>i</code> and <code>j</code> and is greater than <code>d</code>.

 MODULE      GCD.
 IMPORT      Integers.
  
 PREDICATE   Gcd : Integer * Integer * Integer.
 Gcd(i,j,d) <- 
            CommonDivisor(i,j,d) &
            ~ SOME [e] (CommonDivisor(i,j,e) & e > d).
  
 PREDICATE   CommonDivisor : Integer * Integer * Integer.
 CommonDivisor(i,j,d) <-
            IF (i = 0 \/ j = 0)
            THEN
              d = Max(Abs(i),Abs(j))
            ELSE
              1 =< d =< Min(Abs(i),Abs(j)) &
              i Mod d = 0 &
              j Mod d = 0.