code: func ValidateChange(key string, f ValueChangeValidationFunc) schema.CustomizeDiffFunc {
  	return func(d *schema.ResourceDiff, meta interface{}) error {
  		old, new := d.GetChange(key)
  		return f(old, new, meta)
  	}
  }
docs: // ValidateChange returns a CustomizeDiffFunc that applies the given validation
  // function to the change for the given key, returning any error produced.