Subject: Support for virtual targets

From: Jan Niklas Böhm <[email-masked]>
Date: Sat, 16 Apr 2022 21:13:50 +0200

Hello,

I was wondering why this implementations lists under its notes [1] 
(third point) that:

	- empty targets are considered always out of date

This caught me a bit by surprise and I was wondering whether its 
possible to actually check whether the target is ood by looking at its 
dependencies, similar to how redo-python does it.  The use case is also 
given in those documents, e. g. compiling a latex document [2].

 From what I was able to gather the corresponding lines are probably l. 
130 and/or l.158 in ood.go.  I am going off of the most recent version 
1.24.0.  If I can help with this, please let me know, although I am 
afraid that I have not written anything in Go so far.

Cheers
Nik

[1]: http://www.goredo.cypherpunks.ru/Notes.html
[2]: 
https://redo.readthedocs.io/en/latest/cookbook/latex/#virtual-targets-side-effects-and-multiple-outputs