Indicate how accuracy is treated over add/subtract operations in DV_QUANTIFIED types

Gerke Geurts:
The DV_AMOUNT and DV_ABSOLUTE_QUANTITY classes contain (optional) accuracy attributes. However the specifications for the outcome of mathematical operations (especially the add/subtract operators) seem to be silent on the accuracy of the results of these operations.

A further question that then arises, is what to do with values for which the accuracy is +/-0. The OpenEhr spec reserves this values as indicating that the accuracy has not been specified. I take it that the outcome of any mathematical operation on a value with unrecorded/unknown accuracy must have an unrecorded/unknown accuracy itself. However, if that is the case, the calculation of formulas that have constants (which have a true accuracy 0) becomes an issue, because the OpenEhr spec currently does not seem to allow for the representation of values with a +/-0 accuracy.

Rong:
While we are on this, maybe we should consider how two precisions (optional attribute of DV_QUANTITY) can be combined as well.