% 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

\Umathcode`a="7"4"1D44E
\Umathcode`b="7"4"1D44F
\Umathcode`x="7"4"1D465
\Umathcode`y="7"4"1D466
\Umathcode`A="7"4"1D434
\Umathcode`B="7"4"1D435
\Umathcode`C="7"4"1D436
\Umathcode`P="7"4"1D443
\Umathcode`3="7"4`3
\Umathcode`+="2"4`+
\Umathcode`=="3"4`=
\Umathcode`(="4"4`(
\Umathcode`)="5"4`)

\Udelcode`(="4`(
\Udelcode`)="4`)

$$\Uradical "4 "221A {x}$$
$$\Uroot    "4 "221A {3}{x^3+y^3}$$

$$
  \Udelimiterover  "4 "23DE {a+b}
+ \Udelimiterunder "4 "23DF {a+b} = C
$$

$$
  \Umathaccent    "0 "4 "23DE {a+b}
$$

$$
A \mathrel{\Uoverdelimiter  "4 "2192 {a+b}}
B \mathrel{\Uunderdelimiter "4 "2192 {a+b}}
$$

$$
{a \overwithdelims() b}
$$


\centerline{$ a \overwithdelims() b $}

$$
P(a)
$$

\bye