InfoPath Dev

I have a textbox that is set with the following: concat(my:string, xdXDocument:GetDOM("LineBreaks")/linebreaks/newLine, "text on new line") LineBreaks is a xml included as a ressource with the following content:  <?xml version="1.0"?> <linebreaks>      ...

I have a form library on my site. It contains one form.xml. When  user A opens this form, should the form be opened in the browser. This works. If user B makes a try to open the form when user A has the form open, would I have that user B is prevented to open the form. How can I do this? 

Because check-out functionality not is user-friendly and form in read-only can be submitted to my webservice, I need a solution for only one user at the time can have the form open. The form is in a Form library.  Is this possible?