message: added StructureElement.set
diff: @@ -31,6 +31,12 @@ namespace StructureElement {
  return { kind: 'element-location', unit: unit!, element: element || (0 as ElementIndex) };
  }
  + export function set(a: StructureElement, unit?: Unit, element?: ElementIndex): StructureElement {
  + if (unit) a.unit = unit
  + if (element !== undefined) a.element = element
  + return a;
  + }
  +
  // TODO: when nominal types are available, make this indexed by UnitIndex
  export type Set = SortedArray<ElementIndex>
  