Vim allows highlight groups with no name to be created; but doesn't
support getting any information for them. Two commas in a row in a
"syn match" creates a new highlight group that's named "". This bug is
triggered by the distributed doxygen.vim.
syn match doxygenStartSkip2 +^\s*\*$+ contained
\ nextgroup=doxygenBody,doxygenStartSpecial,,doxygenStartSkip skipwhite skipnl
^^