dsl: fixed expression handling

dsl/README.md

@@ -92,3 +92,24 @@ or configuration statements, and look like this:
 * [ ] const parse
 * [ ] live reload
 * [ ] serialize modified code back to original indentation
+
+## implementation notes
+
+### `DslExp` trait behavior
+
+this is the trait which differentiates "a thing" from
+"a thing that is many things".
+
+|source         |key|exp    |head     |tail           |
+|---------------|---|-------|---------|---------------|
+|`a`            |`a`|e0     |`a`      |None           |
+|`(a)`          |e1 |`a`    |`(a)`    |None           |
+|`a b c`        |e2 |e0     |`a`      |`b c`          |
+|`(a b c)`      |e1 |`a b c`|`(a b c)`|               |
+|`(a b c) d e`  |e1 |e3     |`(a b c)`|`d e`          |
+|`a (b c d) e f`|e1 |e0     |`a`      |`(b c d) e f`  |
+
+* e0: Unexpected 'a'
+* e1: Unexpected '('
+* e2: Unexpected 'b'
+* e3: Unexpected 'd'