The left and right commands can
find the left and right endpoints of an interval.
(See Section 8.2.3, Section 6.3.6,
Section 6.5.1, Section 9.3.4 and Section 9.3.5 for
other uses of left and right.)
left and
right take one argument:
I, an interval.
left(I) returns the left endpoint of the interval
I.
right(I) returns the right endpoint of the interval
I.
