procedure main()

stddev() # reset state / empty
every  s := stddev(![2,4,4,4,5,5,7,9]) do
   write("stddev (so far) := ",s)

end

procedure stddev(x)  # running standard deviation
static X,sumX,sum2X

   if /x then {   # reset state
      X := []
      sumX := sum2X := 0.
      }
   else {         # accumulate
      put(X,x)
      sumX +:= x
      sum2X +:= x^2
      return sqrt( (sum2X / *X) - (sumX / *X)^2 )
      }
end