it does a saturating add rather than a normal add.  This ensures the
probability value stays exactly <= 1.  Previously, floating point
rounding error could cause it to be slightly above 1 and would therefore
cause some asserts to misfire during debugging mode.

the feature extractor.  Also added the necessary input validation
to make sure this feature doesn't get misused.

added some tests to make sure the code really does work with the
relaxed interface.

a copy by value.

for the assignment_function and sequence_labeler consistent.

computed.

missing asserts.

when the user uses the sequence label rejection feature.