Training mode

[fmang]

What if there existed a mode where every time a note is missed, the song is automatically rewinded by a few seconds to replay the missed part.

To be smarter, the game could wait until the combo ends or until there is a tiny break before rewinding abruptly.

When to resume could also be smartly computed by resuming at the previous combo, so that the context is somewhat preserved. Replaying a few notes out of nowhere won't help.