eql.rb

class Obj 
  attr_reader :a

  def initialize(a)
    @a = a
  end
  
  def ==(other)
    a == other.a
  end

  alias_method :eql?, :==

  def hash 
    a.hash
  end
end

o1 = Obj.new(:a)
o2 = Obj.new(:a)
o3 = Obj.new(:a)

a = [[o1], [o2], [o3]]
p a.inject(:&) # Recognizes and returns the common element

#------------------------------------

class Obj 
  attr_reader :a

  def initialize(a)
    @a = a
  end
  
  def ==(other)
    a == other.a
  end
end

o1 = Obj.new(:a)
o2 = Obj.new(:a)
o3 = Obj.new(:a)

a = [[o1], [o2], [o3]]
p a.inject(:&) # Does not recognize common elements.. returns []