triangle($pos = top, $color = red, $size = 20px 10px)
  if $pos in (top right bottom left)
    width: 0;
    height: 0;
    display: block;
    border-style: solid;
    if $pos in (top bottom)
      border-width: $size;
    else if $pos in (right left)
      border-width: $size[1] $size[0];
    border-color: transparent;
    border-{$pos}-width: 0;
    border-{opposite-position($pos)}-color: $color