A binary function from quantities to quantities and an extension of the arithmetic subtraction operation to ScalarIntervals (q.v.) generally. When applied to two arithmetically-comparable scalars, MINUEND and SUBTRAHEND, difference yields the scalar that is the result of subtracting SUBTRAHEND from MINUEND. For example, (difference 88 11) is 77 and (difference (kilogram 4.2) (kilogram 3)) is (kilogram 1.2).

Note that when MINUEND and SUBTRAHEND are not arithmetically-comparable, (difference MINUEND SUBTRAHEND) is undefined. For example, (difference (minute 1) (meter 3)) is undefined, since (minute 1) is a time and (meter 3) is a distance, and time-quantities are not comparable with distances.

For addition, see Plus Fn.