# File narray_ext.rb, line 334
  def covariance(x,y,*ranks)
    x = NArray.to_na(x) unless x.kind_of?(NArray)
    x = x.to_type(NArray::DFLOAT) if x.integer?
    y = NArray.to_na(y) unless y.kind_of?(NArray)
    y = y.to_type(NArray::DFLOAT) if y.integer?
    n = x.rank_total(*ranks)
    xm = x.accum(*ranks).div!(n)
    ym = y.accum(*ranks).div!(n)
    ((x-xm)*(y-ym)).sum(*ranks) / (n-1)
  end