Problem with username including the 'at' char

We are installing and configuring SVN for the first time, thus sorry if
the question sounds stupid: we want our users to use svnserve throw ssh
ie type commands in the form

svn ls svn+ssh://sshusername@stuff/stuff/stuff

the trouble is that our ssh usernames are actually ldap usernames and,
for other reasons, are in the form 'a@b'.
Typing

svn ls svn+ssh://a@b_at_stuff/stuff/stuff

gives a bad URL error from SVN. The first idea was to write it like in
an scp command

svn ls svn+ssh://'a_at_b'@stuff/stuff/stuff

but again a bad URL error. Single-quoting the whole URL and escaping the
single quotes seems to freak the ssh out...
Any suggestions? Thanks in advance.

-- 
Elia Cogodi <eliacogodi@tin.it>
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