def sorted_real_alpha_values(fractions)
      fractions.compact.collect { |a| a >= 0 && a.finite? ? a : nil}.compact.sort
    end
def sorted_real_alpha_values(fractions)
      fractions.compact.collect { |a| a >= 0 && a.finite? ? a : nil}.compact.sort
    end
Gives a sorted array of the directional fractions given, taking
 into account that some of them may be nil, negative, or even -INFINITY,
 and thus needs to be excluded before sorting.

 @param [Array<Float, NilClass>] fractions a collection of alpha values
 @return [Array<Float>] sorted (valid) alpha values