Labelled slurs

[Timothy Lanfear]

When slurs overlap, I noticed that I could omit one of the slur labels. 
Can I count on this behaviour or did I just get lucky? Taking the 
example from the NR 1.3.2:

\version "2.22.0"
\fixed c' {
  <c~ f\=1( g\=2( >2 <c e\=1) a\=2) >
  <c~ f( g\=2( >2 <c e) a\=2) >
}

--
Timothy Lanfear, Bristol, UK.