When a pattern variable is defined on only one side of a disjunction, #100
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When a pattern variable is defined on only one side of a disjunction,
it is now an error to use it again later in the pattern, or in a value expression. The reason is that the semantics are too confusing. If you get this error, just rename the pattern variable to avoid the conflict, or use a wildcard instead.
This is technically a breaking change, but I'm guessing that it will occur only rarely in practice. Therefore I am only bumping the minor version number.
Fixes #99