TODO: parser validity checks

[tnelson]

Using this issue as a TODO list for parser checks.

• If a TField appears with an unused variable name, error.
• If a TField appears with an invalid field, error.
• All relations used must be declared either remote or local.
• No overlap between local/remote/inc/out relations

[tnelson]

Arity mismatches between table use and table declaration. (This WILL cause +/- rules to be ignored otherwise!)

[tnelson]

Any head vars that don't appear in body, including fields.

This is a controversial language choice, since it involves deciding whether every field needs to be explicitly constrained in rules...

[tnelson]

If attempting to modify new.locSw in a forward rule, should be an automatic parser failure.