el_bst *usun(el_bst * drzewo, el_bst *x)
{
    el_bst *y=x->p, *z;
    if((x->left) && (x->right))
    {
        z=(rand()%2) ? usun(root, poprzednik(x)) : usun(root, nastepnik(x));
        z->left=x->left; if(z->left) z->left->p=z;
        z->right=x->right; if(z->right) z->right->p=z;
    }
    else z=(x->left) ? x->left : x->right;

    if(z) z->p=y;
    if(!y) root=z;
    else if(y->left == x) y->left =z;
    else y->right=z;
    return x;
}