Making markup functions parametric

[Urs Liska]

In the following markup command definition

#(define-markup-command (test layout props enclosure content)
  (scheme? markup?)
   (interpret-markup layout props (markup #:circle content)))

I would like to make the #:circle parametric, i.e. I want to pass
something into the function (e.g. as the 'enclosure' argument) and
apply the corresponding markup function within the markup expression.
The argument could be a function directly or a symbol from which it is
referenced.

Howeve, I would like to dynamically apply the markup function here
rather than create a structure that chooses a complete markup
expression based on the desirec markup type.

Unfortunately I don't really have an idea what "#:circle" actually
*is*, so I have no clue about getting where I need to.

BestUrs