bizarre behaviour of `output-filename`

[Werner LEMBERG]

Consider this input file called `c.ly`:

```
output-filename = "foo"
output-suffix = "bar"

\book {
  \paper {
    output-suffix = "baz"
  }
  { c' }
}

\book {
  { c' }
}

\book {
  { c' }
}
```

The created output files are

  foo-baz.pdf
  c-bar.pdf
  c-bar-1.pdf

I consider it bizarre that the top-level variable `output-filename` is
only considered in a `\book` block where the paper variable
`output-suffix` is also set, contrary to `output-suffix`.  Why is
`output-filename` considered at all?  Where in the code is this read?
AFAIK, top-level variables are read with `ly:parser-lookup`, and I
can't find this for `output-filename`.

Or is this an inheritance thing?  Does a `\book` block inherit all
top-level expressions and convert them to paper variables?


    Werner