code: func (pe PathExpression) pushBackOneKeyLeg(key string) PathExpression {
  	newPe := PathExpression{
  		legs:  append(pe.legs, pathLeg{typ: pathLegKey, dotKey: key}),
  		flags: pe.flags,
  	}
  	if key == "*" {
  		newPe.flags |= pathExpressionContainsAsterisk
  	}
  	return newPe
  }
docs: // pushBackOneKeyLeg pushback one leg of KEY type