[definitions] Redefine expression-equivalent per ISO directives

To be replaceable in context, and add a clarifying example.

Author: CaseyCarter

source/lib-intro.tex

@@ -227,16 +227,25 @@
 
 \definition{expression-equivalent}{defns.expression-equivalent}
 \indexdefn{expression-equivalent}%
-relationship that exists between two expressions \tcode{E1} and \tcode{E2} such
-that
-\begin{itemize}
-\item \tcode{E1} and \tcode{E2} have the same effects,
-
-\item \tcode{noexcept(E1) == noexcept(E2)}, and
+expressions that all have the same effects,
+either
+are all potentially-throwing\iref{except.spec} or
+are all not potentially-throwing,
+and
+either
+are all constant subexpressions or
+are all not constant subexpressions
 
-\item \tcode{E1} is a constant subexpression if and only if \tcode{E2} is a
-constant subexpression
-\end{itemize}
+\begin{example}
+For a value \tcode{x} of type \tcode{int}
+and a function \tcode{f} that accepts integer arguments,
+the expressions
+\tcode{f(x + 2)},
+\tcode{f(2 + x)},
+and
+\tcode{f(1 + x + 1)}
+are expression-equivalent.
+\end{example}
 
 \definition{handler function}{defns.handler}
 \indexdefn{function!handler}%