code: public static double spearmanRankCorrelationCoefficient(IntegerVector a, 
                                                              IntegerVector b) {
          // NOTE: should this code ever be on the critical path, we should
          // re-implement it to operate on the Vector instances themselves
          return spearmanRankCorrelationCoefficient(a.toArray(), b.toArray());
      }
docs: Computes the Spearman rank correlation coefficient for the two {@code
  IntegerVector} instances.
  
  <p> This implementation properly accounts for ties according to the
  procedure specified in <i>Nonparametric Statistics for The Behavioral
  Sciences</i> by Sidney Siegel and N. John Castellan Jr. Second
  Ed. (1988).
  
  @throws IllegalArgumentException when the length of the two vectors are
  not the same.