Add mjs_detachBody to C API.

PiperOrigin-RevId: 626947850
Change-Id: I7c897048098ecd4f2247725ef2d378dbd00ef4a0

src/user/user_model.h

@@ -158,12 +158,14 @@ class mjCModel : public mjCModel_, private mjSpec {
  public:
   mjCModel();
   mjCModel(const mjCModel& other);
-  mjCModel& operator=(const mjCModel& other);
-  mjCModel& operator+=(const mjCModel& other);
   ~mjCModel();
   void CopyFromSpec();  // copy spec to private attributes
   void PointToLocal();
 
+  mjCModel& operator=(const mjCModel& other);  // copy other into this, if they are not the same
+  mjCModel& operator+=(const mjCModel& other);  // add other into this, even if they are the same
+  mjCModel& operator-=(const mjCBody& subtree);  // remove subtree and all references from model
+
   mjSpec spec;
 
   mjModel* Compile(const mjVFS* vfs = nullptr);  // construct mjModel
@@ -196,6 +198,9 @@ class mjCModel : public mjCModel_, private mjSpec {
                                    std::map<mjCDef*, int>& def_map,
                                    const std::vector<mjCDef*>& defaults);
 
+  // delete from list the elements that are compatible with other but not this model
+  template <class T> void RemoveFromList(std::vector<T*>& list, const mjCModel& other);
+
   // delete elements marked as discard=true
   template <class T> void Delete(std::vector<T*>& elements,
                                  const std::vector<bool>& discard);