code: def calculate_base_sequence(base_offsets):
      
      return ''.join([ chr(c) for c in list(itertools.chain.from_iterable(
                        [ range(ord(x), ord(y) + 1) for (x, y) in base_offsets ])) ])
docs: Return a sequence of characters corresponding to the specified base.
  
      Example::
  
          >>> calculate_base_sequence([ ('0', '9'), ('A', 'F') ])
          '0123456789ABCDEF'
  
      @param base_offsets: a list of character offsets in the form of tuples
             ``(x, y)`` where ``x`` corresponds to the first character of a
             sequence and ``y`` to the last character of this sequence.
  
      @return a sequence of characters, i.e., a string, corresponding to the
              specified base.