/* Find first/last of equal keys according to son_id. */
idx = P(fill_stack) (stack+i+1, max_depth-i-1,
P(tree_son) (stack[i].buck, son_id), TREE_KEY(), son_id);
/* Find first/last of equal keys according to son_id. */
idx = P(fill_stack) (stack+i+1, max_depth-i-1,
P(tree_son) (stack[i].buck, son_id), TREE_KEY(), son_id);