// ?= would allow us to define the images global before @importing 
// the file if we were to distribute this

images ?= '/images'

image(path)
  url(join('/', images path))

// or

image(path)
  url(images + '/' + path)

body
  background image('foo.png')

// functions can act as both mixins
// or have a return value

image(path)
  if mixin
    background image(path)
  else
    url(images + '/' + path)

body
  image('something.png')
  background image('something.png')