Write some amount of documentation

docs/selection.md

@@ -0,0 +1,62 @@
+# Selection
+
+Cycle through values over time for evolving patterns.
+
+## Step Cycle
+
+`cycle` cycles through values based on step runs:
+
+```forth
+60 64 67 3 cycle note  ( cycle through C, E, G )
+```
+
+Each time the step runs, it picks the next value.
+
+## Pattern Cycle
+
+`pcycle` cycles based on pattern iteration:
+
+```forth
+60 64 67 3 pcycle note  ( change note each pattern loop )
+```
+
+## Emit-Time Cycle
+
+`tcycle` creates a cycle list resolved at emit time, useful with `.!`:
+
+```forth
+60 64 67 3 tcycle note 3 .!  ( emit C, E, G in sequence )
+```
+
+## Examples
+
+Rotating bass notes:
+
+```forth
+"bass" s
+  c3 e3 g3 b3 4 cycle note
+.
+```
+
+Evolving pattern over loops:
+
+```forth
+"lead" s
+  0.5 1.0 0.75 0.25 4 pcycle gain
+.
+```
+
+Arpeggiated chord:
+
+```forth
+"pluck" s
+  c4 e4 g4 c5 4 tcycle note 4 .!
+```
+
+## Words
+
+| Word | Stack | Description |
+|------|-------|-------------|
+| `cycle` | (v1..vn n -- val) | Cycle by step runs |
+| `pcycle` | (v1..vn n -- val) | Cycle by pattern iteration |
+| `tcycle` | (v1..vn n -- list) | Create cycle list for emit-time |