def find_license_header_index(src_file_content,
                              license_info,
                              top_lines_count):
    """
    Returns the line number, starting from 0 and lower than `top_lines_count`,
    where the license header comment starts in this file, or else None.
    """
    for i in range(top_lines_count):
        license_match = True
        for j, license_line in enumerate(license_info.prefixed_license):
            if i + j >= len(src_file_content) or license_line.strip() != src_file_content[i + j].strip():
                license_match = False
                break
        if license_match:
            return i
    return None