global

onto the collection. (Currently unused.)
Enumerable, compute the inverse of +vector+ that maps this Enumerable
If +vector+ maps the matching elements of another collection onto this

def inverse_vector(a, vector)
  inverse = a.dup
  (0...vector.size).each do |i|
    inverse[vector[i]] = i unless vector[i].nil?
  end
  inverse
end