def get_common_introduced(db_entry, arches):
    """Returns the common introduction API level or None.

    If the symbol was introduced in the same API level for all architectures,
    return that API level. If the symbol is not present in all architectures or
    was introduced to them at different times, return None.
    """
    introduced = None
    for arch in arches:
        introduced_tag = 'introduced-' + arch
        if introduced_tag not in db_entry:
            return None
        if introduced is None:
            introduced = db_entry[introduced_tag]
        elif db_entry[introduced_tag] != introduced:
            return None
        # Else we have the symbol in this arch and it's the same introduction
        # level. Keep going.
    return introduced