NR 3.2.6 Table of contents: example of \tocItemWithDotsMarkup with custom tocItemMarkup

[Federico Bruni]

http://www.lilypond.org/doc/v2.19/Documentation/notation/table-of-contents

At the bottom of the page there is this part:

"""
Dots can be added to fill the line between an item and its page number:

\header { tagline = ##f }
\paper {
 tocItemMarkup = \tocItemWithDotsMarkup
}
"""

which is useful, but I think that it would be better to show how a 
custom tocItemMarkup can use that function. IIUC, the only (?) way is 
redefining the variable:

\paper {
 tocItemMarkup = \markup { \fill-line { \fromproperty #'toc:text 
\fromproperty #'toc:page } }
 tocItemMarkup = \tocItemWithDotsMarkup
}

This may be obvious for a programmer who knows the concept of variable, 
but not obvious for a musician.