def make_index(self):
for prev, block in zip(self.blocks[:-<NUM_LIT:1>], self.blocks[<NUM_LIT:1>:]):<EOL><INDENT>if not block.is_comment:<EOL><INDENT>self.index[block.start_lineno] = prev<EOL><DEDENT><DEDENT>
Make the index mapping lines of actual code to their associated
        prefix comments.