(VAR x y z )
(RULES 
        or(true, y) -> true
        or(x, true) -> true
        or(false, false) -> false
        mem(x, nil) -> false
        mem(x, set(y)) -> =(x, y)
        mem(x, union(y, z)) -> or(mem(x, y), mem(x, z))
        
)