% start out with plain.tex,
% having TFM-based CM fonts preloaded in \fam 0...3

% load OT math font in \fam 4
\input luaotfload.sty
\font\4={name:XITS Math:mode=base;script=math} at 10pt
\font\5={name:XITS Math:mode=base;script=math;ssty=1} at 7pt
\font\6={name:XITS Math:mode=node;script=math;ssty=2} at 5pt
\textfont4=\4 \scriptfont4=\5 \scriptscriptfont4=\6
\nopagenumbers

%\tracingall
$$
  \Umathchardef\sum="1"4"2211
  \Umathchardef\prod="1"4"220F
  \Umathchardef\sumx="7"4"2211
  \Umathchardef\prodx="7"4"220F
  \Umathchardef\a="7"4`o
  \Umathchardef\w="7"4`O
% \Umathaccent               "0 "4 "23DE {\sum} \quad
% \Umathaccent         fixed "0 "4 "23DE {\sum} \quad
% \Umathaccent bottom        "0 "4 "23DF {\sum} \quad
% \Umathaccent bottom  fixed "0 "4 "23DF {\sum} \quad
% \Umathaccent both          "0 "4 "23DE       "0 "4 "23DF {\sum} \quad
% \Umathaccent both    fixed "0 "4 "23DE fixed "0 "4 "23DF {\sum} \quad
  \Umathaccent overlay       "0 "4 "0338 {\sum} \quad
% \Umathaccent overlay fixed "0 "4 "0338 {\sum} \quad
  \Umathaccent overlay       "0 "4 "20D2 {\sum} \quad
% \Umathaccent overlay fixed "0 "4 "20D2 {\sum} \quad
  \Umathaccent overlay       "0 "4 "0338 {\prod} \quad
  \Umathaccent overlay       "0 "4 "20D2 {\prod} \quad
  \Umathaccent overlay       "0 "4 "0338 {\a} \quad
  \Umathaccent overlay       "0 "4 "20D2 {\a} \quad
  \Umathaccent overlay       "0 "4 "0338 {\w} \quad
  \Umathaccent overlay       "0 "4 "20D2 {\w} \quad
\textstyle
  \Umathaccent overlay       "0 "4 "0338 {\sum} \quad
  \Umathaccent overlay       "0 "4 "20D2 {\sum} \quad
  \Umathaccent overlay       "0 "4 "0338 {\sumx} \quad
  \Umathaccent overlay       "0 "4 "20D2 {\sumx} \quad
  \Umathaccent overlay       "0 "4 "0338 {\prodx} \quad
  \Umathaccent overlay       "0 "4 "20D2 {\prodx} \quad
$$
\bye

