Refinement with ID declared later in letrec causes internal error

[jpolitz]

Both of these programs (with rec or letrec) cause an internal error.

rec n :: Number%(is-zero) = 10
rec is-zero = lam(x): x == 0 end


letrec n :: Number%(is-zero) = 10,
  is-zero = lam(x): x == 0 end:
  n
end

The expected behavior is this error:

Probably refinements assuming the IDs are safe letrec IDs when they are not guaranteed to be.