code: private void backtrack() {
          IAtom qatom = stack[sptr].atom;
          --sptr;
          if (qatom != null) {
              --numMapped;
              avisit[amap[qatom.getIndex()]] = false;
              amap[qatom.getIndex()] = UNMAPPED;
          } else if (sptr != 0) {
              backtrack();
          }
      }
docs: Pops a stack frame until a the query/mol atom pairing is unmapped
  or we reach the bottom of the stack