<..plain,latex math..>+
\def\:temp#1#2{\a:mthpl\o:mathpalette:{#1}{#2}\b:mthpl}
\HLet\mathpalette=\:temp
\NewConfigure{mathpalette}[2]{\c:def\a:mthpl{#1}\c:def\b:mthpl{#2}}
\Configure{mathpalette}{}{}
<.phantom and smash.>
-_-_-
The following is to avoid empty pictures.
<..phantom and smash..>
\def\:temp{\relax
\ifmmode \expandafter\o:mathpalette:\expandafter\mathph@nt
\else \expandafter\makeph@nt \fi}
\HLet\ph@nt=\:temp
\def\:temp{\relax
\ifmmode \expandafter\o:mathpalette:\expandafter\mathsm@sh
\else \expandafter\makesm@sh \fi}
\HLet\smash=\:temp
-_-_-
Before we hade for non-pic
However, the first parameter need not be a singlton macro, so we had failures with that code.