code: Mappings matchRoot(IAtom root) {
          checkCompatibleAPI(root);
          IAtomContainer mol = root.getContainer();
          if (query.getAtomCount() > 0 && ((IQueryAtom) query.getAtom(0)).matches(root)) {
              DfState local = new DfState(state);
              local.setRoot(root);
              return filter(new Mappings(query, mol, local), query, mol);
          } else {
              return new Mappings(query, mol, Collections.<int[]>emptySet());
          }
      }
docs: Match the pattern at the provided root.
  
  @param root the root atom of the molecule
  @return mappings
  @see Mappings