tempo

tempo_to_warp :: Signal.Tempo → Signal.Warp
tempo_to_warp = Signal.integrate Signal.tempo_srate . Signal.map_y (1/)
    . Signal.clip_min min_tempo

with_warp :: (Monad m) => (Warp → Warp) → DeriveT m a → DeriveT m a
d_stretch :: (Monad m) => TrackPos → DeriveT m a → DeriveT m a
d_at :: (Monad m) => TrackPos → DeriveT m a → DeriveT m a
local_to_global :: (Monad m) => TrackPos → DeriveT m TrackPos

d_sub :: TrackPos → TrackPos → TrackLang.CallId → Derive.EventDeriver
d_sub start dur call_id = do
    begin <- local_to_global start
    end <- local_to_global (start + dur)
    d_at begin (d_stretch ((end-begin) / block_dur) (d_block block_id))

• Events can find their time ranges with local_to_global, but can also compute absolute ranges with global_to_local.
• Or d_seq [with_warp (const initial_warp) attack, release]
• So derivers are free to interpret warp as the will, just as they are free to set relative or absolute control signals.
• Examples: trill adds integral number of cycles, grace notes remain constant (i.e. tempo-less), "humanize" function warps existing beats in a semi-random way.
• Warping another signal is g(f^-1(t)), i.e.

      control :: Score → Y
      warp :: Score → Real
      invert warp :: Real → Score
      control `compose` invert warp :: Real → Y
      

  If compose uses at_linear I don't have to worry about a low sample rate becoming obvious through stretching.