var rework = require('rework'),
var reworkFunction = require('rework-plugin-function');
var css = rework(css)
  .use(reworkFunction({ black: black }))
  .toString()
function black(opacity) {
  return 'rgba(0, 0, 0, ' + opacity + ')';
}

var css = rework(css)
  .use(reworkFunction({
    subtract: function(a, b) { return a - b },
    multiply: function(a, b) { return a * b },
    divide: function(a, b) { return a / b },
    floor: Math.floor
  }))
.toString()