'Associates the given xBlock object with the given usage_key.
Arguments:
usage_key (UsageKey) - Usage key of the given xBlock.  This
value is passed in separately as opposed to retrieving
it from the given xBlock since this interface is
agnostic to and decoupled from the xBlock interface.
xblock (XBlock) - An instantiated XBlock object that is
to be stored for later access.'

def _add_xblock(self, usage_key, xblock):

    self._xblock_map[usage_key] = xblock
