def permute(s):
    if len(s)==0:
        return ['']
    head,tail=s[0],s[1:]

    tail_permutation=permute(tail)

    if head.isalpha():
        u,l=head.upper(),head.lower()
        return map(u.__add__, tail_permutation) + map(l.__add__, tail_permutation)
    else:
        return map(head.__add__, tail_permutation)