out.css

body {
  foo: bottom;
  foo: right;
  foo: bottom right;
}

sass

@function opposite-position($pos)
  @if $pos == top
    @return bottom
  @else if $pos == bottom
    @return top
  @else if pos == left
    @return right
  @else if pos == right
    @return left
  @else
    @warn 'Invalid position ' + pos
    @return none

@function opposite($positions)
  $ret: ()
  @each $pos in $positions
    $pos: opposite-position(pos)
    $ret: if(length($ret) == 0, $pos, join($ret, $pos))

body
  foo: opposite(top)
  foo: opposite(left)
  foo: opposite(top left)

stylus

opposite-position(pos)
  if pos == top
    bottom
  else if pos == bottom
    top
  else if pos == left
    right
  else if pos == right
    left
  else
    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)