code: func ValidateValue(key string, f ValueValidationFunc) schema.CustomizeDiffFunc {
  	return func(d *schema.ResourceDiff, meta interface{}) error {
  		val := d.Get(key)
  		return f(val, meta)
  	}
  }
docs: // ValidateValue returns a CustomizeDiffFunc that applies the given validation
  // function to value of the given key, returning any error produced.
  //
  // This should generally not be used since it is functionally equivalent to
  // a validation function applied directly to the schema attribute in question,
  // but is provided for situations where composing multiple CustomizeDiffFuncs
  // together makes intent clearer than spreading that validation across the
  // schema.