-- ewm xs:L n a:n > L n — exponential moving average with smoothing factor a.
--
-- Recurrence: ewm[0] = xs[0]; ewm[i] = a*xs[i] + (1-a)*ewm[i-1].
-- The classic IIR smoother. Higher a tracks the input more closely;
-- lower a gives heavier smoothing. Boundary cases: a=0 freezes at xs[0];
-- a=1 reproduces the input exactly.
--
-- Replaces the fold-with-state pattern (~25 tokens) with one call.
-- Standard mid-alpha case
half>L n;ewm [1, 2, 3, 4, 5] 0.5
-- Empty input → empty output
empty>L n;ewm [] 0.5
-- Single element seeds verbatim, regardless of alpha
single>L n;ewm [42] 0.3
-- a=0 freezes at the first value
freeze>L n;ewm [1, 2, 3, 4, 5] 0
-- a=1 is identity (full reaction to each new input)
identity>L n;ewm [1, 2, 3, 4, 5] 1
-- run: half
-- out: [1, 1.5, 2.25, 3.125, 4.0625]
-- run: empty
-- out: []
-- run: single
-- out: [42]
-- run: freeze
-- out: [1, 1, 1, 1, 1]
-- run: identity
-- out: [1, 2, 3, 4, 5]