lawbreakers in Text.PrettyPrint.HughesPJ

The comments in HughesPJ say that empty is an identity for <> and $$, but the implementation doesn't satisfy either of these laws. Here are the implementations:

p $$ q = Above p False q

p <> q = Beside p False q

and definitions for empty and isEmpty:

empty = Empty

isEmpty Empty = True isEmpty _ = False

It's an easy fix, just adding these two lines before the current definitions of $$:

Empty $$ q = q p $$ Empty = p

And these before the current definition of <>:

Empty <> q = q p <> Empty = p

- Conal