How to shorten ties?

[Gianluca D.]

Good afternoon to everybody!
I know that the answer to my question is probably somewhere in the manual, but 
I will be grateful to anyone who could help me. 
I've been searching for a way to shorten ties. I've tried with commands such 
as "extra-X-extent" but I couldn't make it work.
I'm using Lilypond version 2.7.37 in Linux.
There's an example of the code I used:

\version "2.7"
\score {
  \new Staff \relative c'' {
    \once \override Staff.Tie #'extra-X-extent = #'(-1 . 0) a2~ a4 r
  }
}

Thank you to everybody!

Gianluca D.