\input{umtest-preamble}
\usepackage{unicode-math}
\setmathfont{xits-math.otf}
\setmathfont[version=bold,Colour=009900]{xits-math.otf}
\begin{document}
\[
(x+y)^{{z+c}^{a+b}}
\]
\mathversion{bold}
\[
(x+y)^{{z+c}^{a+b}}
\]
\end{document}