Make use of MutableLogical's feature to track usages count.
A logical with 1 or less usages can't be contained in
another term, so it's safe to skip unification in this case.
Operations such as 'error' and 'warning' are available on input
node in templates and serve to report issues found during
template application, in contrast with those found during evaluation.
DataFormPrinter is hardcoded to support _term and _subterm
hacks and present the terms in a readable way.
Memoizer is searched for an implementation of
NodePresentable, which helps with visualization of SNode
and SNodeReference.
LogicalUtil defines a bunch of overloaded methods that help dispatch
data form construction based on runtime class of logical variable.
This eliminates the need to use coerce in generator template.
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.
ExpressionLogicalVariable is replaced with ExpressionItem wrapping
the same expression.
Fixes ensure correct application of transformations.
Late expression can't be used as "placeholder" logical item.
This expression is to be used in those rare cases
when a logical var is to be passed to an external method.
Not available within EvaluationSetting (new marker interface).
Normally logical variable (as an instance of LogicalItem)
is only to be used within a logical context.
Logical item value expr only to contain a logical variable.
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.
CopyUtil didn't correctly map logicals nested inside DataForms which are bound to Logicals.
That's because CopyUtil relies on DataForm:variablesCount() and
LogicalDataForm:variablesCount didn't consider the case when bound value is itself DataForm.
Fedor Isakov