Within `defform`, `...+` should be treated like `...` and made to not
refer to a `...+` binding. (The identifier `...+` is bound by
`syntax/parse`, so it's not difficult to end up with a for-label
binding of `...+`.)
In places where module names are typeset, such as `racketmodname`,
there's no way to disable the special treatment of `_` as an
identifier prefix as in `racket`. Even using
`make-element-id-transformer` doesn't work, because the module-name
datum is explcitly disconnected from binding withing `racketmodname`.
