code: func (st *state) clearTemplateArgs(args []AST) {
  	for _, a := range args {
  		st.setTemplate(a, nil)
  	}
  }
docs: // clearTemplateArgs gives an error for any unset Template field in
  // args.  This handles erroneous cases where a cast operator with a
  // forward referenced template is in the scope of another cast
  // operator.