Termination of variable definitions

[David Sumbler]

In starting to experiment with selective compilation using includes
and/or the $(if condition action) structure provided by David Kastrup, I
found that there is something very, very basic that I have never really
understood.

So at the risk of embarrassing myself, I should be grateful if somebody
would explain the following: how does Lilypond recognize the end of a
variable definition?

For instance, an example from the documentation shows: 

violin = \new Staff {
  \relative {
    a'4 b c b
  }
}

cello = \new Staff {
  \relative {
    \clef "bass"
    e2 d
  }
}

What I don't understand is why the definition of 'violin' extends from
'new' to the right brace on line 5.  Why does the definition not extend,
for instance, to the brace on line 12?  Alternatively, why does 'violin'
not equate simply to '\new' or to '\new Staff' or '\new Staff {' ?

David