Use [rewind](<http://ruby-doc.org/core-2.3.1/Enumerator.html#method-i-rewind>)
to restart the enumerator.
ℕ = Enumerator.new do |yielder|
x = 0
loop do
yielder << x
x += 1
end
end
ℕ.next
# => 0
ℕ.next
# => 1
ℕ.next
# => 2
ℕ.rewind
ℕ.next
# => 0