+

(+ &rest numbers)

Returns the sum of its arguments, or 0 with no arguments. The result is an integer when all arguments are integers, a ratio when ratios are involved, and a float if any argument is a float (contagion). Integer results promote to big integers on overflow, on every backend. The sum runs left to right one pair at a time, so the arguments ahead of the first float add exactly and only their sum turns into a float: (+ 1/10 1/5 0.0) is 0.3, not the 0.30000000000000004 of converting each argument first, as in SBCL. A complex argument folds the same way: (+ z (- z) 1.5) over an exact complex z is 1.5.