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Computes the difference in a filter's value compared to the input structure
<Delta name="(&string)" upper="(1 &bool)" lower="(0 &bool)" range="(0 &Real)" filter="(&string)" unbound="(0 &bool)" jump="(see below &Int)" relax_mover="(null &string)"/>
The filter is evaluated at parse time and its internal value (through report_sm) is saved. At apply time, the filter's report_sm is called again, and the delta is evaluated.