Simple Ruby class for an extensionally-specified Equivalence Relation with fast lookup

equivalence_relation.rb

require 'set'
class EquivalenceRelation
  def initialize
    @equivalence_class_by_member = {}
  end
  
  def declare_equivalent(*items)
    combined_equivalence_class = items.map {|item| equivalence_class_of(item)}.inject(Set.new) {|accum,set| accum.union(set)}
    combined_equivalence_class.each {|member| @equivalence_class_by_member[member] = combined_equivalence_class}
  end
  
  def equivalence_class_of(x)
    @equivalence_class_by_member[x] || Set.new([x])
  end
  
  def [](x)
    equivalence_class_of(x) - [x]
  end
end