'abc' begins 'abc'
     'abc' begins 'abc\'
     'abc' begins 'abc/x'
    'abcd' begins 'abcd'
    'abcd' begins 'abcd/'
    'abcd' begins 'abcd\x'
   'abcde' begins 'abcde'
   'abcde' begins 'abcde/'
   'abcde' begins 'abcde/x'
       'x' begins 'x'
       'x' begins 'x/'
       'x' begins 'x/abc'
     'abc' begins 'abc\abcd'
    'abcd' begins 'abcd/abc'
     'abc' begins 'abc\abc'
     'abc' begins 'abc\abcd/abcde'
   'abcde' begins 'abcde\abcd/abc'
    'ab c' begins 'ab c'
    'ab c' begins 'ab c\'
    'ab c' begins 'ab c/abc'
   'ab cd' begins 'ab cd'
   'ab cd' begins 'ab cd\'
   'ab cd' begins 'ab cd\x'
    'ab c' begins 'ab c/abc'
   'ab cd' begins 'ab cd\ab c'
     'a+b' begins 'a+b'
     'a+b' begins 'a+b/'
     'a+b' begins 'a+b\abc'