{- |
Set the number of Haskell threads that can run truly simultaneously
(on separate physical processors) at any given time.

GHC notes: in the current implementation, the value may only be
/increased/, not decreased, by calling 'setNumCapabilities'.  The
initial value is given by the @+RTS -N@ flag, and the current value
may be obtained using 'getNumCapabilities'.
-}