Изменения
→Split
<t1, t2> = split(t.left, k)
t.left = t2
update(vt) r = v
'''return''' <t1, t>
'''else'''
<t1, t2> = split(t.right, k - l - 1)
t.right = t1
update(vt) l = v
'''return''' <t, t2>
