Implement xpath notation in drools constraints

It is required to allow an xpath notation in drools constraints like the following:

Person( $pet : /pets ) 

that is formally equivalent to the more verbose:

Person( $pets : pets )
$pet : Pet() from $pets

It should be also possible to have conditions filtering the collection like in:

Person( $pet : /pets[ age == 5 ] )

corresponding to:

Person( $pets : pets )
$pet : Pet(age == 5) from $pets

Of course also multiple indirections has to be allowed, so the following:

Man( $toy : /wife/children/toys )

will be equivalent to:

$man: Man( $wife: wife )
$child: Child() from $wife.children
$toy: Toy() from $child.toys

In this case the xpath notation will also allow to flatten the 2 from nodes in a single one that will internally flatten the collection of collections. Since it is necessary to distinguish the case when we want to iterate a collection from the case when we want to bind the collection as a whole it is possible to mix the collection iteration operator '/' and the simple dereferencing operator '.' as in:

Man( $toys : /wife/children.toys )

In this last case this pattern binds each List<Toy> for each child as a whole, so this is equivalent to:

$man: Man( $wife: wife )
Child( $toys: toys ) from $wife.children