Reverted Item::isSolid redefinable

Author: conradbock

sysml.library/Systems Library/Items.sysml

@@ -20,7 +20,6 @@ standard library package Items {
 	private import SequenceFunctions::includes;
 	private import SequenceFunctions::union;
 	private import ControlFunctions::forAll;
-	private import ScalarValues::Boolean;
 
 	abstract item def Item :> Object {
 		doc
@@ -101,12 +100,11 @@ standard library package Items {
 			 */
 		}
 
-		attribute isSolid: Boolean {
+		attribute isSolid = isEmpty(voids) {
 			doc
 			/*
 			 * An Item is solid if it has no voids.
 			 */
-			attribute :>> self: Boolean = isEmpty((that as Item).voids);
 		}
 
 		abstract item subitems: Item[0..*] :> items, subobjects {