ds051.hs:6: Warning: Pattern match(es) are overlapped in the definition of function `f1':
	    f1 "ab" = ...

ds051.hs:11: Warning: Pattern match(es) are overlapped in the definition of function `f2':
	    f2 ('a' : ('b' : [])) = ...

ds051.hs:16: Warning: Pattern match(es) are overlapped in the definition of function `f3':
	    f3 "ab" = ...