However I don't see much that can be made "unambiguous" in the condition:
Code:
else while ($calc($1 $iif($r(0,1),-,+) $r( [ %rxecart ] )) & $calc($2 - $r( [ %ryecart ] ))) { }
else while ($calc(A +/- B) & $calc(C - D)) { }
else while (X bitwise Y) { }


...besides of course (blind guess):
Code:
else {
  while (condition) { }
}


Edit: I suppose it's the "else while", for the following code will produce the error, and adding { } arround the else statement fixes it:
Code:
//var %x = 1 | if (!%x) noop | else while (%x < 3) { echo -sg %x | inc %x }