def determine_engine(content):
    """
    Determine the engine used in the given content.

    The engine is determined by the ``@startXXX`` used at the beginning of the
    file. Possible values are:

    ::

        @startuml --> plantuml
        @startdot --> graphviz
        @startditaa --> ditaa

    :param str content: Content to analyze.
    :return: The name of the engine found, or None.
    :rtype: str
    """
    if content.startswith('@startuml'):
        return 'plantuml'
    if content.startswith('@startdot'):
        return 'graphviz'
    if content.startswith('@startditaa'):
        return 'ditaa'
    return None
