opposite-position(pos) {
  return bottom if pos == top;
  return top if pos == bottom;
  return right if pos == left;
  return left if pos == right;
  error('Invalid position ' + pos)
}
 
opposite(positions) {
  for pos in positions {
    pos = opposite-position(pos);
    ret = ret is defined ? ret pos : pos;
  }
}
 
body {
  foo: opposite(top);
  foo: opposite(left);
  foo: opposite(top left);
}