def adder(n)
  v = 0                         # v is lexically scoped here
  lambda { v += n }             # v and n are accessed from the closure
end

add1 = adder(10)
p add1.call  #=> 10
p add1.call  #=> 20
p add1.call  #=> 30

add2 = adder(5)
p add2.call  #=> 5
p add2.call  #=> 10
p add2.call  #=> 15