Like this:

in the Scheme programming language you would write

(define (add a b) (+ a b))

in my language you would write

.(add).. a b

number

.(+).. a b

to define subtraction declaratively in terms of "add" you would write the following

.(sub).. a b

number

.(add).. () b a

this says "to subtract b from a find the quantity that added to b equals a"

genetic algorithms are used as a mechanism for finding the quantity in question