def f(x):l=len;m=filter;y=sorted([''.join(m(str.isalnum,i))for i in x],key=l);*z,=m(l,[''.join(i for i in y if-~j==l(i))for j in range(l(y[-1]))]);y==z and+print(*z,sep='\n')or f(z)
for x in (['This is a sample text,', 'that you will have to use to build stairs.', 'The wood may be of excellent quality,', 'or pretty crappy almost falling apart and filled with termites.', 'Bla bla bla - some more text', 'Ok, that will do'],['A', 'small', 'one', 'that', 'contains', 'equal', 'length', 'strings', 'for', 'the', 'special', 'rule'],['Test,', 'test.', 'This', 'is', 'a', 'test.']):
    try:
        f(x)
    except:
        pass

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