code: func (t *Template) Parse() error {
  	if t.program != nil {
  		return nil
  	}
  	program, err := parser.Parse(t.Input)
  	if err != nil {
  		return err
  	}
  	t.program = program
  	return nil
  }
docs: // Parse the template this can be called many times
  // as a successful result is cached and is used on subsequent
  // uses.