Add some properties of [split3]
Add some properties of [split3]

These are probably the most useful "general" properties of
[split3]. They talk about what happens when you try to index, append,
or get length of a [split3]d list.

FStarLang/FStar,FStarLang/FStar,A-Manning/FStar,FStarLang/FStar,A-Manning/FStar,A-Manning/FStar