Fixed manual for interval arithmetic (thanks to Pétiard François)

manual/mpman-app-numbersystems.tex

@@ -68,7 +68,7 @@ and  the right extrema
 \begin{center}\index{interval\_get\_right\_endpoint?\texttt{interval\_get\_right\_endpoint}}\label{Dintervalright}\texttt{r := interval\_get\_right\_endpoint(}\textit{interval}\texttt{);}\end{center}
 and it's always $l\leq r$\,; a number \texttt{x} is always seen as the interval \texttt{[x,x]}, i.e. a set with one element.
 At first glance, interval arithmetic can be confusing: if $w$ is an interval,
-$w*w$ is the set $\left\{ x*x: x\in w\right\}$, but $f(w)=w^2$  is the set $\left\{f(x): x\in w\right\} = \left\{x^2: x\in w\right\}$:
+$w*w$ is the set $\left\{ x*y: x\in w, y\in w\right\}$, but $f(w)=w^2$  is the set $\left\{f(x): x\in w\right\} = \left\{x^2: x\in w\right\}$:
 if $w=[-1,1]$ then $w*w=[-1,1]$ and $w^2=[0,1]$. On the other hand, it's easy to verify that $w*w*w = w^3$ always. For this reason,
 given that currently the
 implementation of the MetaPost functions in MPFI mimics those of MPFR and they are not adapted to the interval arithmetic,