PROLOG Program to Find Area of Circle

constants
    text="Radius of circle is="
    pi=3.14159
    double=2
predicates
    output(integer, real)
    
clauses
    output(0, _),
    output(N, R) if N>0,
        C=2*pi*R,
        A=pi*R*R,
        write(text, R, "Circumference=", C, "Area=", A),
        n1,
        R1=double*R,
        N1=N-1,
        output(N1, R1)