add example to tree addressing peg doc

[jackfoxy]

Add example to tree addressing peg doc using dig by to calculate address in (map (map).

[jackfoxy]

@sigilante @dozreg-toplud

[dozreg-toplud]

@jackfoxy,
I don't think this is a good example: outer/inner etc. are not defined, and +dig:by is two chapters later in hoon.hoon than +peg.

What would you like the example to show/explain? If it was up to me I would try to demonstrate that +peg is associative and +1 is an identity. The latter is already demonstrated, and the former isn't really. Also another example of +peg/+cap interaction wouldn't hurt.

[jackfoxy]

This is the exact use case that got me involved with this.
No double-hashing, no key comparisons, just hops to an index in a mip using dottar (.*).
This works when you have 100K mips all with the same index structure.
This is a practical application of peg.
Pretty sure I've seen precedence for examples that are not fully expanded (so long as they are comprehensible) and most people use the docs as a reference, not as a book.
I dunno. IMO expanding the example would make it harder to follow.