Dear Wolfgang, 

thanks for your mail, that led me to the source in syst-aux where I found the TeX approach, my final solution.

\def\startCitation%
  { \startnarrower[left,right]
    \symbol[leftquotation]
    \removeunwantedspaces}

\def\stopCitation#1\par%
  { \removeunwantedspaces
    \symbol[rightquotation]
    #1
    \stopnarrower}
  
\starttext
  
\startCitation
  \input knuth
\stopCitation
\footnote{This is a Quote of Donald Knuth}

\input knuth

\stoptext

> On 26 Oct 2016, at 18:40, Wolfgang Schuster <schuster.wolfgang@gmail.com> wrote:
> 
>> Michael Eidenbenz <mailto:eidenbenz@arch.ethz.ch> 26. Oktober 2016 um 16:30
>> after some readings in the TeX book I came up with the following solution that works for me without having to change all my instances of startstopCitation in the text.
> This doesn’t work when you have more than one paragraph in the Citation environment.
>> \definestartstop
>> [Citation]
>> [
>> before={\hangindent=20mm\hangafter=0\symbol[leftquotation]},
>> after={\removeunwantedspaces\symbol[rightquotation]},
>> ]
>> 
>> \starttext
>> \startCitation
>> \input tufte
>> \stopCitation
>> \footnote{This is a Quote of Donald Knuth}
>> 
>> \input tufte
>> \stoptext
> Here is a alternative version but you need a empty line or \par after \stopCitation.
> 
> \define\startCitation
>   {\startnarrower[left]
>    \symbol[leftquotation]\ignorespaces}
> 
> \define\stopCitation
>   {\BeforePar{\removeunwantedspaces\symbol[rightquotation]}%
>    \AfterPar {\endgraf\stopnarrower}%
>    \GetPar}
> 
> \starttext
> 
> \startCitation
>   \input knuth
> \stopCitation
> \footnote{This is a Quote of Donald Knuth}
> 
> \input knuth
> 
> \stoptext
> 
> Wolfgang
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