adjusting stem length

[Jean-marc LEGRAND]

Hi list !

Working on XP with lily 2.4.2

I have to adjust the stem length of several notes beamed together.

I've tried :

\once \override Stem #'beamed-lengths = #'(4) a8 [b c d e f]

but it doesn't do anything, whereas

 \override Stem #'beamed-lengths = #'(4) a8 [b c d e f]

is OK.

Is there a way to make sort of a  \override Stem #'beamed-lengths = ##f after 
the notes to go back
to a normal and automatic stem length ?

Another thing is that I don't quite understand the syntax of this  \override 
Stem
#'beamed-lengths...

Thanks for your help !

Regards