After doing an SVN ( subversion ) sparse checkout how can I checkout more files

Question

Say I have a repository as such

- trunk
    - dir0
        - a.txt
        - b.txt
    - dir1
        - c.txt
        - d.txt

then I do a sparse checkout so I only have

- trunk
    - dir1
        - c.txt
        - d.txt

checked out. I decide I actually do need trunk/dir0/a.txt as well How do I tell SVN to add it to the checkout so that my final checkout will be

- trunk
    - dir0
        - a.txt
    - dir1
        - c.txt
        - d.txt