** Computation as Simulation**

It is useful to view computation as simulation,
* cf.*: * isomorphism of semigroups*.[Preparata, F. P. and
Yeh, R. T., * Introduction to Discrete Structures*, Addison-Wesley, 1973,
p. 129.]

Given two semigroupsGand_{1}= [S, °]G, an invertible function_{2}= [T, * ]φ : S → Tis said to be anisomorphismbetweenGand_{1}Gif, for every_{2}aandbinS,φ (a ° b) = φ (a) * φ (b)

from which:
* a ° b = φ ^{-1} ( φ (a) * φ (b) ) *

(defun string+ (x y) (princ-to-string ; phi inverse (+ ; + in model space (read-from-string x) ; phi (read-from-string y)))) ; phi >(string+ "2" "3") "5"