s1    s2    <<   <<=    ==   ~==   >>=    >>
    ""    ""   ---    ""    ""   ---    ""   ---
    ""   "a"   "a"   "a"   ---   "a"   ---   ---
    ""   "c"   "c"   "c"   ---   "c"   ---   ---
    ""   'x'   "x"   "x"   ---   "x"   ---   ---
    ""   '2'   "2"   "2"   ---   "2"   ---   ---
   "a"    ""   ---   ---   ---    ""    ""    ""
   "a"   "a"   ---   "a"   "a"   ---   "a"   ---
   "a"   "c"   "c"   "c"   ---   "c"   ---   ---
   "a"   'x'   "x"   "x"   ---   "x"   ---   ---
   "a"   '2'   ---   ---   ---   "2"   "2"   "2"
   "b"    ""   ---   ---   ---    ""    ""    ""
   "b"   "a"   ---   ---   ---   "a"   "a"   "a"
   "b"   "c"   "c"   "c"   ---   "c"   ---   ---
   "b"   'x'   "x"   "x"   ---   "x"   ---   ---
   "b"   '2'   ---   ---   ---   "2"   "2"   "2"
   'c'    ""   ---   ---   ---    ""    ""    ""
   'c'   "a"   ---   ---   ---   "a"   "a"   "a"
   'c'   "c"   ---   "c"   "c"   ---   "c"   ---
   'c'   'x'   "x"   "x"   ---   "x"   ---   ---
   'c'   '2'   ---   ---   ---   "2"   "2"   "2"
   "x"    ""   ---   ---   ---    ""    ""    ""
   "x"   "a"   ---   ---   ---   "a"   "a"   "a"
   "x"   "c"   ---   ---   ---   "c"   "c"   "c"
   "x"   'x'   ---   "x"   "x"   ---   "x"   ---
   "x"   '2'   ---   ---   ---   "2"   "2"   "2"
     2    ""   ---   ---   ---    ""    ""    ""
     2   "a"   "a"   "a"   ---   "a"   ---   ---
     2   "c"   "c"   "c"   ---   "c"   ---   ---
     2   'x'   "x"   "x"   ---   "x"   ---   ---
     2   '2'   ---   "2"   "2"   ---   "2"   ---
    ''    ""   ---    ""    ""   ---    ""   ---
    ''   "a"   "a"   "a"   ---   "a"   ---   ---
    ''   "c"   "c"   "c"   ---   "c"   ---   ---
    ''   'x'   "x"   "x"   ---   "x"   ---   ---
    ''   '2'   "2"   "2"   ---   "2"   ---   ---