RSI as a function
expr - data series formula
count - lookback period
The standard RSI indicator always uses the series of closing prices for its calculations.
This function makes it possible to calculate RSI for any series of values.
Calculation uses the original Welles Wilder formula.
Wilder's exponential smoothing is equivalent to using 2*len-1 in a regular exponential moving average.
This function supports ultra-fast one-pass calculation when used in the Data Section with a non-variable count.