Commit c579b46c authored by Robbert Krebbers's avatar Robbert Krebbers

Avoid use of `solve_proper`.

Due to Coq bug #10480 or #10474 it actually used `Morphisms.solve_proper`
instead of the version of std++. The version in std++ can inherently
not solve this, so I changed it into a manual proof.
parent 556a5992
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