code: public Integer indexOf(Record o) {
          if(o!=null) {
              for(Map.Entry<Integer, Record> e : entries()) {
                  Integer rId = e.getKey();
                  Record r = e.getValue();
                  if(o.equals(r)) {
                      return rId;
                  }
              }
          }
          return null;
      }
docs: Returns the index of the first occurrence of the specified element in this
  Dataframe, or null if this Dataframe does not contain the element.
  WARNING: The Records are checked only for their X and Y values, not for
  the yPredicted and yPredictedProbabilities values.
  
  @param o
  @return