Add attach tag to MJCF.

PiperOrigin-RevId: 651117953
Change-Id: I57b0e53ed44607d479b3785d908e0837c0804cf3

src/user/user_api.cc

@@ -101,6 +101,7 @@ int mjs_attachBody(mjsFrame* parent, const mjsBody* child,
   try {
     *frame_parent += std::string(prefix) + *child_body + std::string(suffix);
   } catch (mjCError& e) {
+    frame_parent->model->SetError(e);
     return -1;
   }
   return 0;
@@ -116,6 +117,7 @@ int mjs_attachFrame(mjsBody* parent, const mjsFrame* child,
   try {
     *body_parent += std::string(prefix) + *child_frame + std::string(suffix);
   } catch (mjCError& e) {
+    body_parent->model->SetError(e);
     return -1;
   }
   return 0;
@@ -158,6 +160,14 @@ void mj_deleteSpec(mjSpec* s) {
 
 
 
+// add spec (model asset) to spec
+void mjs_addSpec(mjSpec* s, mjSpec* child) {
+  mjCModel* model = static_cast<mjCModel*>(s->element);
+  model->AppendSpec(child);
+}
+
+
+
 // delete object, it will call the appropriate destructor since ~mjCBase is virtual
 void mjs_delete(mjsElement* element) {
   mjCBase* object = static_cast<mjCBase*>(element);
@@ -466,6 +476,14 @@ mjSpec* mjs_getSpec(mjsBody* body) {
 
 
 
+// find spec (model asset) by name
+mjSpec* mjs_findSpec(mjSpec* s, const char* name) {
+  mjCModel* model = static_cast<mjCModel*>(s->element);
+  return model->FindSpec(name);
+}
+
+
+
 // get default
 mjsDefault* mjs_getDefault(mjsElement* element) {
   mjCModel* model = static_cast<mjCBase*>(element)->model;