code: private int[] map(int u, int v, int[] us, int[] mapping) {
          for (int i = 0; i < us.length; i++)
              us[i] = mapping[us[i]];
          return us;
      }
docs: Transforms the neighbors {@code us} adjacent to {@code u} into the target
  indices using the mapping {@code mapping}. The transformation accounts
  for an implicit hydrogen in the query being an explicit hydrogen in the
  target.
  
  @param u       central atom of tetrahedral element
  @param v       mapped central atom of the tetrahedral element
  @param us      neighboring vertices of u (u plural)
  @param mapping mapping from the query to the target
  @return the neighbors us, transformed into the neighbors around v