(VAR x y )
(RULES 
        f(el(x),y) -> f(x,el(y))
        f(x,y) ->= f(l(x),y)
        f(x,y) ->= f(x,r(y))
        l(el(x)) ->= el(l(x))
        el(r(x)) ->= r(el(x))
)