A combination of ` map` and ` reduce` can provide a great deal
of power in a compact form.

The * Euclidean distance* between two
points in *n*-space is the square root of the sum of squares of
the differences between the points in each dimension.

Using ` map` and ` reduce`, we can define Euclidean distance
compactly for any number of dimensions:

(defun edist (pointa pointb) (sqrt (reduce '+ (mapcar 'square (mapcar '- pointa pointb)))) ) >(edist '(3) '(1)) 2.0 >(edist '(3 3) '(1 1)) 2.8284271247461903 >(edist '(3 4 5) '(2 4 8)) 3.1622776601683795