DOUBLEEXPONENTIALSMOOTHING function takes a Geo Time Series™ and smooths it with the
additive Double Exponential Smoothing aka Holt's linear trend method formula.
It consumes three parameters from the top of the stack:
- A trend smoothing factor beta (a numeric parameter between 0 < beta < 1)
- A level smoothing factor alpha (a numeric parameter between 0 < alpha < 1)
- The GTS to smooth
The resulting smoothed GTS and levels are pushed onto the stack (in this order in the LIST).