subtract
Take exactly the evidence named by provenanceRefs off propositionId, leaving the rest of its evidence, grounding and source ids alone.
Passing no refs reads and returns the proposition unchanged. A default over subtractFoldedEvidence with empty grounding and source ids; implementors only need to provide that one.
Return
the proposition as the subtraction left it, or null when the store holds no proposition under that id, which a caller reads as "somebody deleted it".
Parameters
proposition
the proposition to subtract from
provenance
evidence keys or bare locator keys naming what goes