How to show execute another use case from specified step?

[Larr Goneg]

Hi, folks!
suppose in a use case we have to redirect to another use case, not from the beginning, but from some step later, how to show it in sequence diagram?
this is not include, I think.

[Pradeep bhatt]

Use the extend relationship. Extends means that under certain circumstance a use case uses another use case.

[Larr Goneg]

Thanks for reply!
my question is, under some condition, one use case will execute only part of the other use case.
for example, there are two use case,
A:1.do a1
2.do a2
3.go to use case B step 2
B:1.do b1
2.do b2
3.do b3
how should I show it in my sequence diagram?
Mayby from use case good practice point of view, we should modify our use case so that such situation not happen.
Thanks!

[Pradeep bhatt]

I would suggest you break down your sequence diagram.