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I wanted this for use in codegen work: in particular, as part of
generating error messages.

Note the comment on the case for unions.  I still go back and forth on
this topic; see some recent previous commit messages on this branch.
(I think I'm slowing coming around to "unions act like maps", and it's
just that most library code chooses to jump over them transparently,
because that's the least-odd path for resolving NodeBuilder questions;
but it remains the case that none of this is settled for sure yet.)