I'd like to be able to write a function that makes it easy to set one or
more keys in a hash, without worrying about whether the hash is mutable or
not.  It could be called in any of the following ways:

(define imm-h (hash 'a 1))
(define mut-h (make-hash '((a . 1))))

(safe-hash-set imm-hash 'x 2)       ; returns (hash 'a 1 'x 2)
(safe-hash-set imm-hash 'x 2 'y 7) ; returns (hash 'a 1 'x 2 'y 7)

(safe-hash-set mut-hash 'x 2)        ; returns (begin (hash-set! mut-h 'x
2) mut-h)
(safe-hash-set mut-hash 'x 2 'y 7)  ; returns (begin (hash-set! mut-h 'x 2)
(hash-set! mut-h 'y 7) mut-h)

This definition isn't quite what I need:
(define (safe-hash-set h  . args)
    (->* (hash?) () #:rest (non-empty-listof any/c) hash?)
    ...)

I want to be able to say "the rest argument is a list that has an even
number of elements and more than zero elements", but I'm not sure how to
capture that.  Thoughts?

Dave

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