Nested loops

#include "share/atspre_staload.hats"

#define N 4

fun do_something (i: int, j: int): void = println! ("(", i, ", ", j, ")")

implement main0 () = let
    var i: int = 0
    var j: int = 0
in
    for (i := 0; i < N; i := i + 1)
    begin
        for (j := 0; j < i; j := j + 1)
        begin
            do_something (i, j);
        end;
    end
end