code: func (p *parser) expectOneOf(exp1, exp2 ItemType, context string) item {
  	token := p.next()
  	if token.typ != exp1 && token.typ != exp2 {
  		p.errorf("unexpected %s in %s, expected %s or %s", token.desc(), context, exp1.desc(), exp2.desc())
  	}
  	return token
  }
docs: // expectOneOf consumes the next token and guarantees it has one of the required types.