Each equation was written using the same labels (X,Y,Z), but internally they have separate sets, (1:X and 2:X, etc.).

The moral of the story for me: Use more Lua.

Gavin


% Lua counts the spin formulas 
\ctxlua{sfnum = 0} % Count starts at zero
\def\sfID{\ctxlua{context(sfnum)}} % ConTeXt can call the count from Lua to use as an ID for each equation.

% Spin formulas increase the spin formula count and typeset the formula as usual.
\define\startspinformula{
  \ctxlua{sfnum = sfnum + 1}
  \startformula
}
\define\stopspinformula{\stopformula}

% My own \mpos uses the spin formula ID as part of the box label, then sets the contents as math.
\define[2]\mpos{\hpos{\sfID:#1}{$#2$}}

% Two commands for requesting contractions (the connections between spinors), using the spin formula ID.
\define[2]\contract{%
  \startpositionoverlay{graphics}%
    \setMPpositiongraphic{\sfID:#1}{mypos:contract}{to=\sfID:#2}%
  \stoppositionoverlay%
}
\define[2]\contractbelow{%
  \startpositionoverlay{graphics}%
    \setMPpositiongraphic{\sfID:#1}{mypos:contractbelow}{to=\sfID:#2}%
  \stoppositionoverlay%
}