(define (op a b) (cond
 ((eq? a "double") "double")
 ((eq? a "float" ) (cond
                   ((eq? b "double") "double")
                   ((eq? b "float" ) "float")
                   ((eq? b "int"   ) "float"))
 ((eq? a "int"   ) (cond
                   ((eq? b "double") "double")
                   ((eq? b "float" ) "float")
                   ((eq? b "int"   ) "int"))))))

(fold op "int" (list "int" "float" "double"))