streaming.mean.generator =
function()
{
  total = 0
  n = 0

  function(value) {
     total <<- total + value
     n <<- n + 1
  }
}

stream1 = streaming.mean.generator()
stream1(3)
stream1(7)
stream1(1)

mget(c("total", "n"), environment(stream1))


stream2 = streaming.mean.generator()
stream3 = streaming.mean.generator()

stream2(-1)
stream2(-2)
stream3(100)
stream2(-10)