I do not think you should have to pay for December. It is absolutely her responsibility. However this is going to cause your W to explode.

I think this may be a situation to practice being happy over being right.

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M 39
H 35
D5,D4
M 4
T 9
ILYBNILWY 5/18/11
Left 7/11/11
Divorced 12/1/13