Put `TCIf` in `Prop`.

theories/base.v

@@ -67,7 +67,7 @@ as [TCOr (TCAnd P Q) R]: the latter will backtrack to [R] if it fails to
 establish [R], i.e. does not have the behavior of a conditional. Furthermore,
 note that [TCOr (TCAnd P Q) (TCAnd (TCNot P) R)] would not work; we generally
 would not be able to prove the negation of [P]. *)
-Inductive TCIf (P Q R : Prop) :=
+Inductive TCIf (P Q R : Prop) : Prop :=
   | TCIf_true : P → Q → TCIf P Q R
   | TCIf_false : R → TCIf P Q R.
 Existing Class TCIf.