* if -XParallelArrays is given, the symbols for the desugarer are
  taken from 'Data.Array.Parallel' (from whichever package is
  exposed and has the module — the home package is fine, too)
* if -fvectorise is given, the symbols for the vectoriser are
  taken from 'Data.Array.Parallel.Prim' (as above)

(There is one wired in symbol left, namely the data constructor
'base:GHC.PArr.[::]. It'll die another day.)