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Extents
Pairs are also used to store intervals, which represent a range of numbers
from the minimum (the car) to the maximum (the cdr).
Intervals are used to store the X- and Y- extents of printable objects.
For X extents, the car is the left hand X coordinate, and the
cdr is the right hand X coordinate. For Y extents, the car
is the bottom coordinate, and the cdr is the top coordinate.
Procedures for working with intervals are found in scm/lily-library.scm. These procedures should be used when possible to ensure consistency of code.