Note [Builder for a bidirectional pattern synonym]

For a bidirectional pattern synonym, the function 'tcPatToExpr'
needs to produce an /expression/ that matches the supplied /pattern/,
given values for the arguments of the pattern synonym. For example:
  pattern F x y = (Just x, [y])
The 'builder' for F looks like
  $builderF x y = (Just x, [y])

We can't always do this:
 * Some patterns aren't invertible; e.g. general view patterns
      pattern F x = (f -> x)
   as we don't have the ability to write down an expression that matches
   the view pattern specified by an arbitrary view function `f`.
   It is however sometimes possible to write down an inverse;
     see Note [Invertible view patterns].

 * The RHS pattern might bind more variables than the pattern
   synonym, so again we can't invert it
      pattern F x = (x,y)

 * Ditto wildcards
      pattern F x = (x,_)