How to get rid of last comma

Question

I am using the following code to print a list of all characters that say something in a given scene of a play-script.

\newcommand\sceneroles[2][Roles]{%
  \def\scenename{#2}%
  \textbf{#1}:%
  \Bind{?id}{rdf:type}{rl:role}{%
    % \GetVal prints the variable name,
    % \GetValProperty extracts property from the name
    % Set the default left time
    % use the real at:left if it has been set
    \IfProperty{\GetVal{?id}}{is:playsin#2}{%
        \GetValProperty{?id}{rl:name}, %
    }{}%
  }%
}

the problem is, that I cannot get rid of the last comma.

what should be printed is:

Roles: Romeo, Juliet

what is printed:

Roles: Romeo, Juliet,

Answer 1

Use this definition:

\newcommand\sceneroles[2][Roles]{%
  \def\scenename{#2}% Store scene names
  \def\namesep{\def\namesep{, }}% One cycle delay name separator
  \textbf{#1}:%
  \Bind{?id}{rdf:type}{rl:role}{%
    % \GetVal prints the variable name,
    % \GetValProperty extracts property from the name
    % Set the default left time
    % use the real at:left if it has been set
    \IfProperty{\GetVal{?id}}{is:playsin#2}{%
      \namesep\GetValProperty{?id}{rl:name}%
    }{}%
  }%
}

It uses a cunning (La)TeX trick.