@eq operator should produce a warning if used with non-numeric value

The @eq operator is currently intended to be used with numeric values.  But, with a name like 'eq' a rule writer may inadvertently compose a rule with something like "@eq foo" ... which is unlikely to be what the rule writer intends.

It might be helpful if use of @eq with a non-numeric value produced at least a warning message at rule-load time.
