type Indicator a = Bar → (Maybe a )

Indicator takes a bar b and returns an indicator value for
that bar. .....bar is associated with five basic fields:
high, low, open, close price, and transaction volume

......

It is very common while defining indicators to use past
values of an indicator. To support this, we have a combina-
tor (♯) which enables relative indexing into past data. Given
a bar (time), while an indicator, say high , evaluates to the
high price at the bar, high ♯ n yields the high price of n th
previous bar. (♯) is defined as follows:

(♯) :: Indicator a → Integer → Indicator a
ind ♯ n = λ b . ind (b − n )