code 41506;
real procedure CHISQ(X, DF);
value X, DF; real X, DF;
CHISQ:= if DF ≤ 0 then
        STATAL3 ERROR(“CHISQ”, 2, DF)
        else if X ≤ 0 then 0 else
        GAMMA(X, DF / 2, 2);
eop