midpointmode is a global variable. When its value is off, intervals are displayed
as usual (in the form [a;b]).
When its value is on, and if a and b have the same first significant digits,
the interval in displayed in a way that lets one immediately see the common
digits of the two bounds.
This mode is supported only with display set to decimal. In other modes of
display, midpointmode value is simply ignored.
Example 1:
> a = round(Pi,30,RD);
> b = round(Pi,30,RU);
> d = [a,b];
> d;
[3.1415926516056060791015625;3.1415926553308963775634765625]
> midpointmode=on!;
> d;
0.314159265~1/6~e1