code: private static boolean checkSingleAttachment(List<Monomer> monomers, String rGroup, HELM2Notation helm2notation,
  			ConnectionNotation not, InterConnections interconnection, String id) throws AttachmentException {
  
  		for (Monomer monomer : monomers) {
  			/* Are the attachment points there */
  			checkAttachmentPoint(monomer, rGroup);
  
  			/* is the attachment point already occupied by another monomer? */
  			String detail = not.getSourceUnit() + "$" + not.getrGroupSource();
  
  			/* Is the attachment point already occupied by another monomer */
  			/* Intra connections */
  			if (helm2notation.getSimplePolymer(id).getMapIntraConnection().containsKey(detail)) {
  				throw new AttachmentException("Attachment point is already occupied");
  			}
  
  			/* Inter connections */
  			detail = id + "$" + detail;
  
  			/* check */
  			if (interconnection.hasKey(detail)) {
  				throw new AttachmentException("Attachment point is already occupied");
  			}
  
  		}
  		return true;
  	}
docs: method to check for one attachment point the validation
  
  @param monomers
  List of monomers
  @param rGroup
  rGroup of the connection
  @param helm2notation
  HELM2Notation object
  @param not
  ConnectionNotation
  @param interconnection
  InterConnections
  @param id
  @return true if it is valid, false otherwise
  @throws AttachmentException