PROLOG Program to Find Factorial of a Number

predicates
factorial(integer, real)
go
clauses
go if
write("Enter a positive integer number:"),
readint(N),
factorial(N,Result),
write("Factorial of", N, "is=", Result).
factorial(0, 1)
factorial(N, Result) if N>0,
N1=N-1,
factorial(N1, Res),
Result=N*Res.