opposite.styl

out.css

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

sass.rb

module Compass::SassExtensions::Functions::Constants
  # returns the opposite position of a side or corner.
  def opposite_position(position)
    opposite = position.value.split(/ +/).map do |pos|
      case pos
      when "top" then "bottom"
      when "bottom" then "top"
      when "left" then "right"
      when "right" then "left"
      when "center" then "center"
      else
        raise Sass::SyntaxError, "Cannot determine the opposite of #{pos}"
      end
    end
    Sass::Script::String.new(opposite.join(" "), position.type)
  end
end

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)