feat: support nested userset rewrites

internal/check/engine.go

@@ -255,6 +255,11 @@ func (e *Engine) checkUsersetRewrite(
 				Tuple: *r,
 				Type:  expand.ComputedUserset,
 			}, e.checkComputedUserset(ctx, r, c, restDepth)))
+		case *ast.UsersetRewrite:
+			checks = append(checks, checkgroup.WithEdge(checkgroup.Edge{
+				Tuple: *r,
+				Type:  toExpandNodeType(c.Operation),
+			}, e.checkUsersetRewrite(ctx, r, c, restDepth)))
 		}
 	}
 
@@ -263,6 +268,19 @@ func (e *Engine) checkUsersetRewrite(
 	}
 }
 
+func toExpandNodeType(op ast.SetOperation) expand.NodeType {
+	switch op {
+	case ast.SetOperationUnion:
+		return expand.Union
+	case ast.SetOperationDifference:
+		return expand.Exclusion
+	case ast.SetOperationIntersection:
+		return expand.Intersection
+	default:
+		return expand.Union
+	}
+}
+
 func (e *Engine) checkComputedUserset(
 	ctx context.Context,
 	r *RelationTuple,