Add flatten command, accessing destructured elements by index, WIP support for regular expressions

HELP.md

@@ -301,6 +301,10 @@ Example: `[foo, bar, foo]` -> `unique` -> `[foo, bar]`
 Interleave the current destructured subject with the given other destructured subject.
 Example: Say `$a` is `[1, 2, 3]` and subject is `[a, b, c]` -> `zip $a` -> `[a, 1, b, 2, c, 3]`
 
+#### `flatten [<index>]`
+Collapse a nested destructured string down a single level, optionally limiting to a specific `index`.
+Example: `[[a,b], c, [d,e]]` -> `flatten 2` -> `[[a,b], c, d, e]`
+
 
 ### Working with Variables
 
@@ -310,6 +314,8 @@ Assign a variable using the standard syntax: `$a = 'mystr'`
 
 You can then use them anywhere you may use a string: `split $a`
 
+If you variable contains a `:: destructured`, you can access individual elements with their indices: `$a.0`, `$a.1`, &c.
+
 There are also several commands for interactive with variables, defined below.
 
 #### `to <var>`