mid(I) : range -> constantmid(x) : constant -> constant
Parameters:
I is an interval.
x is a real number.
Description:
Returns the middle of the interval I. If the middle is not exactly
representable at the current precision, the value is returned as an
unevaluated expression.
When called on a real number x, mid considers it as an interval formed
of a single point: [x, x]. In other words, mid behaves like the identity.