# File lib/more/facets/math/variance.rb, line 6
  def self.variance(array, &block)
    sum2 = if block_given?
      sum(array){ |i| j = block[i]; j*j }
    else
      sum(array){ |i| i**2 }
    end
    sum2/array.size - mean(array, &block)**2
  end