A variable-arity function from quantities to quantities that is an extension of the arithmetical operation of addition to measurable ScalarIntervals (q.v.) generally. Where ADDEND1, ..., and ADDENDn are pairwise Numerically Comparable (q.v.), (Plus Fn ADDEND1 ... ADDENDn) is the arithmetic sum of ADDEND1, ..., and ADDENDn. For example, (Plus Fn 2 3 4) is 9, and (Plus Fn (meter 1.5) (meter 0.7)) is (meter 2.2).

Note that the term (Plus Fn ADDEND1 ... ADDENDn) is Undefined if there are ADDENDi and ADDENDj (1 <= i,j <= n) that are not comparable. For example, (Plus Fn (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 subtraction, see difference.