(VAR x y z )
(RULES 
        :(x,x) -> e
        :(x,e) -> x
        i(:(x,y)) -> :(y,x)
        :(:(x,y),z) -> :(x,:(z,i(y)))
        :(e,x) -> i(x)
        i(i(x)) -> x
        i(e) -> e
        :(x,:(y,i(x))) -> i(y)
        :(x,:(y,:(i(x),z))) -> :(i(z),y)
        :(i(x),:(y,x)) -> i(y)
        :(i(x),:(y,:(x,z))) -> :(i(z),y)
        
)