ds051.hs:7:1: warning: [-Woverlapping-patterns (in -Wdefault)]
    Pattern match is redundant
    In an equation for ‘f1’: f1 "ab" = ...

ds051.hs:12:1: warning: [-Woverlapping-patterns (in -Wdefault)]
    Pattern match is redundant
    In an equation for ‘f2’: f2 ('a' : 'b' : []) = ...

ds051.hs:17:1: warning: [-Woverlapping-patterns (in -Wdefault)]
    Pattern match is redundant
    In an equation for ‘f3’: f3 "ab" = ...