code: public QueryAtomGroupImpl pop() {
          QueryAtomGroupImpl group = new QueryAtomGroupImpl();
          boolean first = true;
          for (QueryAtom atom : atoms) {
              if (first) {
                  first = false;
              }
              else {
                  group.addAtom(atom);
              }
          }
          return group;
      }
docs: A convenience method to clone the atom group instance and pop the first atom.
  Only the instance itself will be cloned not the atoms.
  @return A new query instance with the first atom removed.