f [n] = [n]
f [m, n] = [m, n, 0, 1]
f [m, n, r, 1] = [m, n, mod m n, 2]
f [m, n, r, 2] = if r == 0 then [n] else [m, n, r, 3]
f [m, n, p, 3] = [n, p, p, 1]
It needs to be implemented with variable-length lists instead of fixed-length tuples, in order to satisfy the type system.
Do you think it is because other programming languages don't uses the semi-colon that way? Or that periods are usually used to access class members, but not to finish function declarations?
The two clauses separated by a semicolon are sentences; thus the semicolon is logically and grammatically equivalent to a full stop. (As long as you normalise the capitalisation). The difference is in nuance, flow, and other soft things.
So when do you use a semicolon instead of a full stop? Whenever you want to!
The really difficult punctuation mark in English is the full colon.