Indent within a score

[Павел]

How to make indent in the middle of a score (not first system)? In some 
cases, it would be very convenient. Because additional score blocks 
"break" the logic of input and lead to other problems: additional spaces 
between systems, resetting bar numeration, rehearsal marks, etc...

This question has already been asked, but there is no answer:
https://lists.gnu.org/archive/html/lilypond-user/2007-09/msg00528.html
https://lists.gnu.org/archive/html/lilypond-user/2009-06/msg00114.html
https://lists.gnu.org/archive/html/lilypond-user/2010-10/msg00346.html