code: func (t *Template) Exec(ctx *Context) (string, error) {
  	err := t.Parse()
  	if err != nil {
  		return "", errors.WithStack(err)
  	}
  	v := newEvalVisitor(t, ctx)
  	r := t.program.Accept(v)
  	switch rp := r.(type) {
  	case string:
  		return rp, nil
  	case error:
  		return "", rp
  	case nil:
  		return "", nil
  	default:
  		return "", errors.WithStack(errors.Errorf("unsupport eval return format %T: %+v", r, r))
  	}
  }
docs: // Exec the template using the content and return the results