Issue1406.agda:33,24-28
I'm not sure if there should be a case for the constructor refl,
because I get stuck when trying to solve the following unification
problems (inferred index ≟ expected index):
  {_} ≟ {_}
  sing F ≟ sing F₁
when checking that the pattern refl has type HEq (sing F) (sing F₁)
