text: import { createDuration } from './create'
  
  function addSubtract (duration, input, value, direction) {
    var other = createDuration(input, value)
  
    duration._milliseconds += direction * other._milliseconds
    duration._days += direction * other._days
    duration._months += direction * other._months
  
    return duration._bubble()
  }
  
  // supports only 2.0-style add(1, 's') or add(duration)
  export function add (input, value) {
    return addSubtract(this, input, value, 1)
  }
  
  // supports only 2.0-style subtract(1, 's') or subtract(duration)
  export function subtract (input, value) {
    return addSubtract(this, input, value, -1)
  }
  