generic[K, C:Comparator[K]]
struct Tree{
    ...
}

implementation Tree{

    generic[S]
    method find(Self self, S key): bool
    where Comparator[S].compare(K,S)
    {
        var node = self.root;
        while (node != null) {
            switch(self.c.compare(node.key, key)){
            case GT: node = node.right;
            case LT: node = node.left;
            case EQ: return true;
            }
        }
        return false;
    }

}