If I'm making my own data types, one common thing I want to do is make 
smart constructors that double as match expanders. I could use 
`define-match-expander` with two transformers, but then I can't add a 
contract to the smart constructor with `contract-out`. If I try to write 

  [foo (->* (widget?) (#:name symbol?) foo?)]))

(define (make-foo widget #:name [name #f])

(define-match-expander foo
  (syntax-parser [(_ widget) #'(? foo? (app foo-widget widget))])
  (syntax-parser [(_ arg ...) #'(make-foo arg ...)]))

...then the use of `contract-out` hides the fact that `foo` is a match 
expander. What should I do?

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