And I see that this works even if I create another class LegalName with the same shape but a different validation rule - e.g. perhaps a LegalName has to contain a space. If I try to use one in place of the other I get a compile error: "Type 'UserName' is not assignable to type 'LegalName'. Types have separate declarations of a private property 'compute'" Looks like it's effectively doing nominal typing, which I'm liking.
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