unusable backtrace

[Werner LEMBERG]

[782f5528e0, using a self-compiled, static version of Guile 2.2.7]

While working on some LilyPond Scheme code I made a typo in file
`define-markup-commands.scm`.  While executing `lilypond -V` (after
`make install`) to process my `.ly` file I get the following
backtrace.

```
          19 (ly:book-process #<Book> #< Output_def> #< Output_def> #)
          18 (ly:spacing-spanner::set-springs #<Grob SpacingSpanner >)
          17 (ly:separation-item::calc-skylines #<Grob NonMusicalPap…>)
          16 (ly:hara-kiri-group-spanner::pure-height #<Grob Vertic…> …)
          15 (ly:axis-group-interface::adjacent-pure-heights #<Grob …>)
          14 (ly:axis-group-interface::pure-height #<Grob BassFigur…> …)
          13 (ly:axis-group-interface::pure-height #<Grob BassFigur…> …)
          12 (ly:axis-group-interface::adjacent-pure-heights #<Grob …>)
          11 (ly:grob::stencil-height #<Grob BassFigure >)
          10 (ly:text-interface::print #<Grob BassFigure >)
           9 (ly:text-interface::interpret-markup #< Output_def> (…) #)
In ice-9/eval.scm:
   298:42  8 (_ #(#(#<directory (lily) 1900780>) #< Output_def> (…) …))
In unknown file:
           7 (ly:text-interface::interpret-markup #< Output_def> (…) #)
In ice-9/eval.scm:
   293:34  6 (_ #(#(#<directory (lily) 1900780>) #< Output_def> (…) …))
In unknown file:
           5 (ly:text-interface::interpret-markup #< Output_def> (…) #)
In ice-9/eval.scm:
   293:34  4 (_ #(#(#(#(#<directory (lily) 1900780>) #< Out…> …) …) …))
    159:9  3 (_ #(#(#(#(#<directory (lily) 1900780>) #< Out…> …) …) …))
    263:9  2 (_ #(#(#(#(#<directory (lily) 1900780>) #< Out…> …) …) …))
   223:20  1 (proc #(#(#(#(#<directory (lily) 1900780>) #< …> …) …) …))
In unknown file:
           0 (%resolve-variable (7 . arg) #<directory (lily) 1900780>)
```

IMHO, this is completely useless.  There is not a single hint to the
location of the error – why does Guile treat
`define-markup-commands.scm` as an 'unknown file'?

What am I missing?


    Werner