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`tc_to_bool` to turn a type class into a Boolean that expresses if there is an instance

Merged Robbert Krebbers requested to merge robbert/bool_of_tc into master

This MR introduces the following function:

(** Given a proposition [P] that is a type class, [bool_of_tc P] will return
[true] iff there is an instance of [P]. *)
Definition bool_of_tc (P : Prop)
  {p : bool} `{TCIf P (TCEq p true) (TCEq p false)} : bool := p.

It's particularly useful in Ltac, where it's sometimes needed to figure out if an instance exists or not. An example use can be found in https://gitlab.mpi-sws.org/FP/iris-coq/merge_requests/199

Edited by Robbert Krebbers

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