code: private void addChainMultiModel(Structure s, Chain newChain, String transformId) {
  
  		// multi-model bioassembly
  
  		if ( modelIndex.size() == 0)
  			modelIndex.add("PLACEHOLDER FOR ASYM UNIT");
  
  		int modelCount = modelIndex.indexOf(transformId);
  		if ( modelCount == -1)  {
  			modelIndex.add(transformId);
  			modelCount = modelIndex.indexOf(transformId);
  		}
  
  		if (modelCount == 0) {
  			s.addChain(newChain);
  		} else if (modelCount > s.nrModels()) {
  			List<Chain> newModel = new ArrayList<>();
  			newModel.add(newChain);
  			s.addModel(newModel);
  		} else {
  			s.addChain(newChain, modelCount-1);
  		}
  
  	}
docs: Adds a chain to the given structure to form a biological assembly,
  adding the symmetry expanded chains as new models per transformId.
  @param s
  @param newChain
  @param transformId