These experimental features are unique to Hugs.
Except of the debugging primitives,
they require the -98 option.
Trex is a very powerful and flexible record system. See Section 7.2 of the “Hugs 98 User Manual” for details.
To use equality and show on extensible records,
a module must import Hugs.Trex.
This module also defines an empty record value and type:
emptyRec :: Rec EmptyRow