code: public Report compare(Xml a, Xml b) {
          return compare(a, b, null, null);
      }
docs: This method is used to compare two XML {@code Xml} objects with each other. The differences are written to
  the {@code Report} object. This method uses the default arbitrator and reporter.
  BFS (breadth-first search) comparison is used when comparing the XML trees.
  The default arbitrator {@code DefaultArbitrator} is very strict and will not accept any differences.
  @param a the {@code Xml} object to compare with
  @param b the {@code Xml} object to compare with
  @return @{code Report} object