SMM11 — Should a binary measure have a formula next to a functor ?

In SMM, a binary measure has a functor.
Examples in the spec (instance models) suggest functor values like “sum”, “difference”, “times”, “divide”.
This sounds like functor should have an enumeration defined in the spec, like also accumulator has for a collective measure.
However, SMM does not do that for functor. Why not? It is rather similar (see also that accumulator has a value “sum” ..).
On the other hand: an enum can be too rigid, as it is not easily extendable. As Alain Picard has admitted, it would for vendors be more flexible to also allow formula’s (like rescaled measures have), for binary measures, e.g. “A * B” (instead of functor “times” to multiple A and B).
But that would require that a binary measure should have a formula next to a functor ?

The RTF discussed this issue, along with issues 17483 and 17503 and found a resolution which which is consistent with the resolution of both the other issues.
Per resolution of issue 17483, a BinaryMeasure, as DimensionalMeasure, has an optional formula.
Per resolution of issue 17503, a BinaryMeasure has an Operation (as customFunctor), if and only if its functor is custom. For this Operation the context is a measurand and a pair of base measurements.
Per Resolution of issue 17483, also a RescaledMeasure, as DimensionalMeasure, has an optional formula. Consistent with the resolutions for both CollectiveMeasure (per issue 17483) and BinaryMeasure (per issue 17503), the RTF then proposes to further revise RescaledMeasure, as follows:
• Add “offset” and “multiplier” to RescaledMeasure. For example, F = (C*9/5)+32. The addend (e.g. 32) is the offset and 9/5 is the multiplier.
• RescaledMeasure has an operation if and only if its multiplier and offset are not given. The context of it a measurand and a base measurement.