Introduce TermOperator helper and refactor execute()s to use replaceSubjectMatchingTerm

src/vm/commands/clear.ts

@@ -17,6 +17,6 @@ export class Clear extends Command<{}> {
     }
 
     execute(vm: StrVM): Awaitable<StrVM> {
-        return vm.tapInPlace(ctx => ctx.replaceSubjectAsString(''))
+        return vm.replaceContextMatchingTerm({ override: '' })
     }
 }