Starting with 5240116f3cdf169e3aa226d9f8206a5f5b99c867 we only support
GHC versions 8.4.4 and up (more precisely, the GHC version associated
with stackage lts-12 and up). The imports in question are now in
Prelude and need not be imported explicitly.
This layout modifier is useful for the case if you desire to add a master
pane that has fixed width (it's fixed even if there is just one window
opened). Especially nice feature if you don't want to have too wide
terminal in a master pane.
The layout is implemented as an addition to Master layout, so it reuses
most of the code.