Restrict other user to access opened web-based InfoPath form

Question

May I know how to restrict other user from accessing a particular web-based InfoPath form in a document library which was open by a user?
(Suppose that the InfoPath form need to be amend from time to time by a group of personnel)

Scenario:
User A open a web-based InfoPath form and make some modification. While User A is still editing the InfoPath form, User B came along and wanted to edit the form. So, is there any way to stop User B from opening the InfoPath form and inform him/her that the form is open by another user? (It would be nice if the message can tell User B that the document is currently open by User A)

Answer 1

Require the item be checked out for editing in the versioning settings.