Optional XmlRule that doesn't break the transformation

[geirolz]

AS user
IW to define an XmlRule that doesn't break the transformation process if target is missing or if it fails
SO we can create a system to define mandatory or not an XmlRule.

We can define a new type OptionalXmlRule and creare an hierarchy with sealed trait BaseXmlRule. Current type XmlRule will inherit from BaseXmlRule and it will mantain his name even if a better name should be MandatoryXmlRule, maybe we can use a type alias.

At this point transformer engine will process all rules pattern matching hover them following the right behavior.

API Syntax proposal

sealed trait BaseXmlRule
case class MandatoryXmlRule extends BaseXmlRule
case class OptionalXmlRule extends BaseXmlRule

type XmlRule = MandatoryXmlRule

val xmlrule : XmlRule = root.missing.node => Append(<foo />)
val optXmlRule : OptionalXmlRule = rule.optional
val mandatoryXmlRule : MandatoryXmlRule = optXmlRule.mandatory