def _addToSegmentUpdates(self, c, i, segUpdate):
<EOL>if segUpdate is None or len(segUpdate.activeSynapses) == <NUM_LIT:0>:<EOL><INDENT>return<EOL><DEDENT>key = (c, i) <EOL>if key in self.segmentUpdates:<EOL><INDENT>self.segmentUpdates[key] += [(self.lrnIterationIdx, segUpdate)]<EOL><DEDENT>else:<EOL><INDENT>self.segmentUpdates[key] = [(self.lrnIterationIdx, segUpdate)]<EOL><DEDENT>
Store a dated potential segment update. The "date" (iteration index) is used
later to determine whether the update is too old and should be forgotten.
This is controlled by parameter ``segUpdateValidDuration``.

:param c: TODO: document
:param i: TODO: document
:param segUpdate: TODO: document