def _clean(self, unit):
return unit.replace(<EOL>'<STR_LIT:/>', '<STR_LIT:_>').replace(<EOL>'<STR_LIT:->', '<STR_LIT>').replace(<EOL>'<STR_LIT:U+0020>', '<STR_LIT>').replace(<EOL>'<STR_LIT:%>', '<STR_LIT>')<EOL>
Clean out special characters from unit abbreviations.