Support `parent` of top-level abstract clafers

[mantkiew]

The following model

abstract FeatureModel
abstract Feature
    [ parent in Feature, FeatureModel]

produces an error

Compile error at line 3 column 7...
Function 'in' cannot be performed on
TClafer {_hi = ["root"]}
in
TClafer {_hi = ["c0_Feature","c0_FeatureModel"]}

the type checker incorrectly assumes that the parent of an abstract clafer is the root.

[mantkiew]

After fixing the compiler (commit), the following model

abstract FeatureModel
abstract Feature
    [ parent in Feature, FeatureModel]


FM : FeatureModel
    F1 : Feature
       F2 : Feature

produces a correct Choco output with a statement

c0_Feature.addConstraint($in(joinParent($this()), union(global(c0_Feature), global(c0_FeatureModel))));

for which Choco errors with

Exception in thread "main" org.clafer.ast.analysis.TypeException: Cannot join abstract c0_Feature . parent

Also, the model produces an incorrect Alloy output

abstract sig c0_Feature
{}
{ (this.~@r_c0_Feature) in (c0_Feature + c0_FeatureModel) }

because r_c0_Feature is not defined. Actually, it should be a derived union of the parents of the concrete clafers:

r_c0_Feature = r_c0_F1 + r_c0_F2

so, the correct Alloy code should be

abstract sig c0_Feature
{}
{ (this.~@(r_c0_F1 + r_c0_F2) in (c0_Feature + c0_FeatureModel) }