Hi,

according to the section "Custom citation renderings" in the bibliography manual, I have specified an own setup for citation rendering.
However, this seems only to work for single citations (of one source). I want to also specify the separator but did not get it to run.

Here is a minimal example:
```
\setupinteraction[state=start]

\startbuffer[testdata]
@Book{knuth1,
	author = {Donald E. Knuth},
	title = {TEX and METAFONT. New directions in typesetting},
	year = {1979},
	publisher = {Addison-Wesley},
}
@Article{someother,
	author = {Some Person and Some Otherperson},
	title = {Another title},
	year = {1900},
}
\stopbuffer

% enable tracing
\enabletrackers[publications, publications.crossref, publications.details, publications.cite, publications.strings]

\usebtxdataset[main][testdata.buffer]
\usebtxdefinitions[aps]
\setupbtx[dataset=main]
\definebtxrendering[bibrendering][aps][dataset=main, numbering=short]
\setupbtxlist[aps][
  alternative=b,
  distance=.5em,
]

\definebtx[aps:cite:special][aps:cite][
	left={((},
	right={))},
	separator:2={sfd},
	separator:3={sfd},
	separator:4={sfd},
]

\startsetups btx:aps:cite:special
	% just a dummy
	\btxcitereference
	\currentbtxtag
\stopsetups

\setupbtx[aps:cite][
	alternative=special,
]

\starttext

Both said something~\cite[knuth1, someother].

\placelistofpublications[bibrendering]

\stoptext
```

The output of this is:

> Both said something ((knuth1someother)).

How can I get the separator to work? My expectation would be:

> Both said something ((knuth1sfdsomeother)).

or

> Both said something ((knuth1sfd someother)).

Best,
Gerion