ruby instance_eval within block

omg.rb

class Baba
  def besta
    puts "o baba e besta"
  end

  def go &block
    xpto = "nada"
    instance_eval &block
  end
end

x = 5
albie = Baba.new
albie.go do |um_baba|
  puts x #=> 5
  puts albie == um_baba #=> true
  puts self == albie #=> true
  puts besta #=> o baba e besta
  puts xpto #=> raise, não existe
end