[armada651]

Wouldn't it be fair to say that it's just a shortened version of "or else"? Just like most programming languages skip "then" in "if ... then" even though it's often required for the sentence to look natural.

[enf]

Algol was not prone to abbreviation. Algol 58 used "or if," so if they had wanted "or else," they would have used "or else."