local XYZ in
X=10
if X>=Y then Z=X else Z=Y end
end
The comparison X>=Y returns true or false, if it can decide which is the case.
If Y is unbound, it cannot decide, strictly speaking.What does it do? Continuing
with either true or false would be incorrect.Raising an error would be a drastic
measure, since the program has done nothing wrong (it has done nothing right
either).We decide that the program will simply stop its execution, without signaling
any kind of error