more clarification and documentation

CHANGELOG.md

@@ -8,6 +8,8 @@ Version 0.1.9.0
 
 <https://github.com/mstksg/emd/releases/tag/v0.1.9.0>
 
+*   *Breaking*: `hlMags` field in `HHT v n a` now contains `n + 1` items,
+    instead of `n` items.  Use `V.init` to regain the original behavior.
 *   Added `hlInitPhase` field to `HHTLine`, to denote the initial phase that an
     HHT Line starts at.
 *   Added `hhtResidual` field to `HHT`, to store the residual of the original

src/Numeric/HHT.hs

@@ -72,6 +72,15 @@ import qualified Math.FFT.Base             as FFT
 -- | A Hilbert Trasnform of a given IMF, given as a "skeleton line".
 data HHTLine v n a = HHTLine
     { -- | IMF HHT Magnitude as a time series.
+      --
+      -- It may be useful to "zip" this vector with 'hlFreqs'.  To do this,
+      -- use a function like 'SVG.init' or 'SVG.tail' to make these two
+      -- vectors contain the same length, or 'weaken'/'shift' to make
+      -- indices in 'hlFreqs' usable as indices in 'hlMags'.
+      --
+      -- Prior to v0.1.9.0, this was a length-n vector, just like
+      -- 'hlFreqs'.  To get the same behavior, use 'SVG.init' on this new
+      -- field's value.
       hlMags      :: !(SVG.Vector v (n + 1) a)
       -- | IMF HHT instantaneous frequency as a time series (between 0 and 1).
       --