Why has `NonMusicalPaperColumn` a width?

[Werner LEMBERG]

Consider this example

```tex
\version "2.19.2"

music = { r4 r r \mark III r }

{ \music }

{ \override Score.RehearsalMark.non-musical = #'()
  \music }

\layout {
  \context {
    \Score
    \override NonMusicalPaperColumn.stencil = #ly:paper-column::print
    \override PaperColumn.stencil = #ly:paper-column::print } }
```

Why does the `NonMusicalPaperColumn` grob have a width of 0.5 units?
Where it is defined?  And where does the additional 0.25 units between
the rest and the non-musical paper column come from?


    Werner