LogicalVariableDataForm generates to either a list node or a term.
This choice can be made based on the structure alone.
Logical variable "array" corresponds to list node, everything else to a term.
Usually one uses Splice with LogicalItemExpression wrapping a logical variable.
Using LogicalDataForm wrapping same variable makes Splice unnecessary.
In order to keep code readable these are wrapped again in a cons list.
In general, one should avoid using bare LogicalDataForm, as these are
indistiguishable from normal LogicalVariable.
LogicalItemValueExpression is to play the same role as valueOf(logical)
within an instance of EvaluationSubject.
LogicalItemExpression is to represent the logical variable itself.
A term may indicate that it "extends" another open term.
An extended term has essentially the same form as the one
being extended, so that they can be unified.
Stronger rules for logical variables scope: constraint rule can't reference logical variable defined in another constraint rule. Logical variable definitions are localized to ConstraintRule template.
Simplify/cleanup the logic of searching for available logical variables.
Fedor Isakov