code: public void addArrayIdentity(String array, String... identities) {
          arrayIdentities.put(array, new ArrayIdentityFields(identities));
      }
docs: Adds a group of fields that can uniquely identify array elements for
  object arrays
  
  @param array The name of the array field
  @param identities The fields of the array element that can identiy an
  element
  
  In the following document:      <pre>
  {
  ...
  "aField": [
  { "_id":1,"field":...},
  { "_id":2,"field":...}
  ]
  }
  <pre>
  the call looks like
  <pre>
  jsonCompare.addArrayIdentity(new Path("aField"),new Path("_id"));
  </pre> If there are more than one fields that uniquely identify an
  eleent, list those in the argument list.