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

src/vm/commands/line.ts

@@ -28,13 +28,10 @@ export class Line extends Command<LineData> {
 
     execute(vm: StrVM, data: LineData): Awaitable<StrVM> {
         // `line <cmd>` is equivalent to `lines` -> `each <cmd>` -> `join`, so just do that
-        return vm.tapInPlace(ctx =>
-            ctx.replaceSubject(async sub =>
-                vm.runInChild(async (child, ctx) => {
-                    await (new Lines).execute(child, {})
-                    await (new Each).execute(child, { exec: data.exec })
-                    await (new Join).execute(child, {})
-                    return ctx.getSubject()
-                })))
+        return vm.replaceContextFromChild(async child => {
+            await (new Lines).execute(child)
+            await (new Each).execute(child, { exec: data.exec })
+            await (new Join).execute(child, {})
+        })
     }
 }