[m-dev.] Types with user defined equality.

[Ralph Becket]

In the fast string module I just posted, it would have been nice
to be able to declare the (structured) str type as having an
equality pred, but have this notion of equality visible only
*outside* the defining module.

As it stands, you have to torture your code (or make everything
cc_multi) if you pattern match against a d.u. type with user-defined
equality, making it far less attractive.  Indeed, searching on the
string "where equality" fails to come up with any instances of its
use in extras or the library (even set.m doesn't use it).

- Ralph
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