Here is the file that I have been working with:

\installlanguage[packed][en]
\setuplanguage[packed][spacing=packed]
\startsetups[tightspace]
  \spaceskip 0.5\interwordspace plus .5\interwordstretch minus \interwordshrink
\stopsetups

\def\dostartbibitem[#1]{\startBibItem[title={#1},reference={#1}]}
\def\startbibitem{\dosingleempty\dostartbibitem}
\def\stopbibitem{\stopBibItem}

\definedescription[BibItem][
width=broad,
margin=1.5pc,
indenting={no},
indentnext=no,
alternative=hanging,
hang=1,
headcommand=\gobbleoneargument,
align=right,
before={\directsetup{tightspace}\bgroup\language[packed]},
after={\egroup},
]

\definereferenceformat [cite]  [type=title,left={},right={}]
\definereferenceformat [bibpage]   [type=page]

\starttext

\startbibitem[A, B, and C]\stopbibitem % TEST by commenting these
%\startbibitem[A{,} B{,} and C]\stopbibitem % lines out

\startbibitem[ABC]\stopbibitem
\startbibitem[A]\stopbibitem
\startbibitem[B]\stopbibitem
\startbibitem[C]\stopbibitem

* \cite[A, B, and C] \qquad{\red output should  be: A{,} B{,} and C}
 
**\cite[ABC] \qquad{\red output should be: ABC}

***\cite[A], \cite[B], \cite[C]  \qquad{\red output should be: A, B, C}

****\cite[A]\cite[B]\cite[C] \qquad{\red output should be: ABC}

\stoptext


On Wed, Feb 22, 2017 at 11:44 AM, Hans Hagen <pragma@wxs.nl> wrote:
On 2/22/2017 4:50 PM, Alan Bowen wrote:
Hi, Hans—

The problem is the line \startbibitem[A, B, and C]\stopbibitem. I can
either keep it and any others like  \cite[A], \cite[B] or
\cite[A]\cite[B] are messed up or I can comment it out and these same
citations work.
\startbibitem[A{,} B{,}and C]\stopbibitem (as you suggested) does allow
the other citations to work. But, then, neither \cite[A, B, and C] nor
the \cite[A{,} B{,} and C] works.

so we need a small example


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