`ly:context-property` and empty alist

[Werner LEMBERG]

The documentation of `ly:context-property` is as follows.

  -- Function: ly:context-property context sym def

     Return the value for property SYM in CONTEXT.  If DEF is given,
     and property value is ‘'()’, return DEF.

I have a use case where I want to say

```
\set SYM = #'()
```

i.e., I explicitly want to override the default DEF with an empty
list.  How can I do that?


    Werner