class MoreMath::Subset

set it applies to the array [ 0, 1, ..., size - 1 ] instead.)
Returns the subset for rank #rank and #collection. (If no collection was

def value
  result = []
  c = @collection || (0...size).to_a
  r = @rank
  0.upto(size) do |i|
    r[i] == 1 and result << c[i]
  end
  result
end