Unique names

[xogeny]

There are many cases where it would be good to guarantee unique names for elements. For this reason, I think it would be good to introduce another special pattern to indicate uniqueness. It could be in the name pattern or it could be in the rule.

The name pattern might seem like a good place (after all, it is the name that we want to be unique). However, then we get into the problem of this special syntax overlapping valid patterns. We already have this problem with _ (i.e. we don't have a way to actually require an underscore...if we ever wanted that).

For this reason, it is probably better to put this in the rule itself. Prefixing a rule name with something like @ might work. There should be a rule that all rules with the same name also have the same value for this uniqueness property.