DEMA (Double Exponential Moving Average) is not just "two EMAs." It's a clever mathematical hack to cancel out the lag inherent in a standard EMA. By subtracting the "error" (the difference between a single EMA and a double EMA) from the original EMA, DEMA produces a curve that hugs the price action much tighter.
Introduced by Patrick Mulloy in the January 1994 issue of *Technical Analysis of Stocks & Commodities*, DEMA was designed to reduce the lag of trend-following indicators. Mulloy realized that smoothing always introduces lag, but by combining single and double smoothing, you could mathematically negate some of that delay.
The "physics" relies on the fact that EMA2 lags EMA1 roughly as much as EMA1 lags the price. Therefore, $2 \times \text{EMA1} - \text{EMA2}$ pushes the value forward, correcting the lag.
DEMA is implemented using two internal `Ema` instances (or equivalent scalar state variables). The calculation is purely algebraic and requires no heap allocations during the `Update` cycle.
