.. efun:: string xor_bits(string str1, string str2)

  :arg:`str1` and :arg:`str2` are both bitstrings. The result of the function is a bitstring with the binary-xor of :arg:`str1` and :arg:`str2`, ie. a string in which a bit is set only if the corresponding bits in either :arg:`str1` or :arg:`str2` (but not both) is set.

  .. usage::

    The below example demonstrates how :efun:`xor_bits` cancels out the bit common to s1 and s2, but retains bits present only in one string::

      string s1, s2, s3;

      s1 = set_bit("", 3);
      s1 = set_bit(s1, 15);  // s1 is "( (", bits 3 and 15

      s2 = set_bit("", 3);
      s2 = set_bit(s2, 4);   // s2 is "8", bits 3 and 4

      s3 = xor_bits(s1, s2); // s3 is "0 (", bits 4 and 15

  .. seealso:: :efun:`clear_bit`, :efun:`set_bit`, :efun:`test_bit`, :efun:`next_bit`, :efun:`last_bit`, :efun:`count_bits`, :efun:`and_bits`, :efun:`or_bits`, :efun:`invert_bits`, :efun:`copy_bits`