Ton,
You have already reported the bug. Thank you. I will try to correct it as soon as possible.
In the meantime, it wouldn't have taken you much longer to write:
ab = a==0&& b==0?0:(a==0?abs(b):(b==0?abs(a):gcd(a,b))); bc = b==0&& c==0?0:(b==0?abs(c):(c==0?abs(b):gcd(b,c))); cd = c==0&& d==0?0:(c==0?abs(d):(d==0?abs(c):gcd(c,d))); g = a==0&& bc==0?0:(a==0?abs(bc):(bc==0?abs(a):gcd(a,bc))); g = ab==0&& cd==0?0:(ab==0?abs(cd):(cd==0?abs(ab):gcd(ab,cd)));
than to write your last post.