T14141.hs:27:1: warning: [-Woverlapping-patterns (in -Wdefault)]
    Pattern match has inaccessible right hand side
    In an equation for ‘f’: f MkD = ...

T14141.hs:42:1: warning: [-Woverlapping-patterns (in -Wdefault)]
    Pattern match is redundant
    In an equation for ‘exhaustive’: exhaustive B = ...