code: static PermutationParity permutation(int[] neighbors) {
          return neighbors.length == 2 ? PermutationParity.IDENTITY : new BasicPermutationParity(Arrays.copyOf(neighbors,
                  neighbors.length - 1));
      }
docs: Create a permutation parity for the given neighbors. The neighbor list
  should include the other double bonded atom but in the last index.
  
  <pre>
  c3
  \
  c2 = c1  = [c3,c4,c1]
  /
  c4
  </pre>
  
  @param neighbors neighbors of a double bonded atom specified by index
  @return a new permutation parity