def visit_module(self, node):
        """
        A interface will be called when visiting a module.

        @param node: node of current module
        """
        if not node.file_stream:
            # Failed to open the module
            return
        isFirstLineOfComment = True
        isDocString = False
        lines = node.stream().readlines()
        for linenum, line in enumerate(lines):
            if line.strip().startswith(b'"""'):
                # This is a simple assumption than docstring are delimited
                # with triple double quotes on a single line.
                # Should do the job for Twisted code.
                isDocString = not isDocString

            if isDocString:
                # We ignore comments in docstrings.
                continue

            matchedComment = COMMENT_RGX.search(line)
            if matchedComment:
                if isFirstLineOfComment:
                    # Check for W9401
                    comment = matchedComment.group()
                    if (comment.startswith(b"#  ") or
                        not comment.startswith(b"# ")):
                        self.add_message('W9401', line=linenum + 1, node=node)
                    # Check for W9402
                    strippedComment = comment.lstrip(b"#").lstrip()
                    if strippedComment:
                        firstLetter = strippedComment[0:1]
                        if (firstLetter.isalpha() and
                            not firstLetter.isupper()):
                            self.add_message('W9402', line=linenum + 1, node=node)
                    isFirstLineOfComment = False
            else:
                isFirstLineOfComment = True