[XeTeX] Overfull \hbox when using inline math scripts

[Daniel Greenhoe]

I have a rather long document involving mathematics that sometimes has
the "Overfull \hbox" problem when I use inline mathematical scripts.
Before I go hacking up the document with newline and \raggedright
commands, is there any more elegant solution currently available?
Below (see also attachment) is an example:

\documentclass[12pt]{book}
\usepackage{fontspec}
\usepackage{unicode-math}
\usepackage{geometry}
\geometry{
  xetex,centering,twoside,noheadfoot,nomarginpar,
  paper=a4paper,margin=20mm,
  showframe
  }
\setmainfont{texgyrepagella-regular.otf}
\setmathfont{xits-math.otf}
\setlength{\parindent}{0pt}
\begin{document}%
\thispagestyle{empty}%
%\sloppy
%\raggedright
Theorem 1.1 (The Theorem That Has This Rather Long Title)
Let the tuple $(X, Y, Z, A, B, C, +, x, -, !, \#)$
be some useful mathematical structure.
Then, \ldots
\end{document}%

Many thanks in advance,
Dan
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