You're supposed to undo the changes that you made to a buffer's
variables when doing filetype-related settings. The variable
'b:undo_ftplugin' contains the commands necessary to revert any changes
that you have made to accomodate this filetype.
I removed the mappings for left and right in a quickfix windows because
I don't use them, and the 'qf' plug-in already maps those (meaning I had
an error when I tried to unmap them after qf already did so).