GHC 8.2.2 and 8.4.3 dislike the following rules:

``` haskell

{-# RULES

"pair fst snd" forall p. (,) (exl p) (exr p) = p

"swap" forall p. (,) (exr p) (exl p) = swap p

 #-}

```

Error messages:

``` haskell

/Users/conal/Haskell/concat/plugin/src/ConCat/Rebox.hs:485:1: warning:

    A constructor, (,), appears as outermost match in RULE lhs.

    This rule will be ignored.

    |

485 | "pair fst snd" forall p. (,) (exl p) (exr p) = p

    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

/Users/conal/Haskell/concat/plugin/src/ConCat/Rebox.hs:489:1: warning:

    A constructor, (,), appears as outermost match in RULE lhs.

    This rule will be ignored.

    |

489 | "swap" forall p. (,) (exr p) (exl p) = swap p

    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

```

GHC 8.0.2 didn't complain about these rules, though I'm unsure whether they worked as desired.

Why disallow such rules (with constructors at the head)?

-- Conal