More radare2 (3/?): binary editing / persisting / debugging

Invoke r2 with -w flag -> Write mode.

In visual mode ('V') the visual assembler 'A' is available, and
allows to enter assembly directly in place of the current seek.
=> patch binary.

Visual mode supports a number of shorcuts (listed in the book,
but also as inline help).  The `;` shortcut, allows to add
comments to the assembly.
Exercise: how to do so in normal mode?


## Flags

It is useful to place flags around, e.g when an interesting
offset is found.

    f myflag = $$    # Where $$ is the 'current seek' variable.


## Persist

I was wondering how to persist metadata.
As it turns out, radare2 can save projects and version them via
git.  Try `P?` and read up :)


## Debug

Visual debugging: `Vpp` (or `V` followed by hitting `p` twice)
Top: stack
Middle: registers
Bottom: instructions

db          List breakpoints
db main     Add breakpoint to main.
dc          Continue to breakpoint.
dsr         Step till end of frame (that is, till return)

F2 | B      toggle breakpoint   (Same as `db $$`)
F4          Run to cursor
F7 | s      Step
F8 | S      Step over
F9          Continue

What is visual diff?