def alpha_min_first_intersection(fractions)
      fractions.compact.collect { |a| a > @alpha_min ? a : nil}.compact.min
    end
def alpha_min_first_intersection(fractions)
      fractions.compact.collect { |a| a > @alpha_min ? a : nil}.compact.min
    end
Gives the minimum value among the directional fractions that is larger
 than @alpha_min (e.g. invalid values like too small values, nil, negative,
 or even -INFINITY are exlcuded).

 @param [Array<Float, NilClass>] fractions a collection of alpha values
 @return [Float] the minimum value among the valid alphas