Factory to create and describe derived variables.
Structure that represents a derivation; can generate values and derived variables from base variables.
This is a temporary structure for grouping elements to be returned from computeValueAndModified, not for permanent storage.
For derived variables, holds Derivations and Factories for producing Derivations. All derived variables are produced by their specific subclass of DerivationFactory. A class interested in generating Derivations will ask various DerivationFactories to instantiate them from a number of VarInfos, and each DerivationFactory determines how many (if any) Derivations are worth generating.
Each Derivation holds a set of base variables as VarInfo. The Derivation can generate: VarInfo of the derived variable it represents; the value of the variable when given a ValueTuple.