In Observation, add the new ImagingSelection resource  (http://build.fhir.org/imagingselection.html) to the resource types able to be referenced in the Observation's derivedFrom attribute.  derivedFrom type would now be: Reference([DocumentReference|http://build.fhir.org/documentreference.html] | ImagingStudy | ImagingSelection | QuestionnaireResponse | Observation | MolecularSequence)

This was discussed with II during the January 20 OnO meeting (request comes from the II/WG20 meeting of March 22).

Also change the description of the derivedFrom attribute from "Related measurements the observation is made from" to "Related resource from which the observation is made" as measurement has no clear meaning as currently written.