docs: def qteSetLexer(self, lexer):
          """
          Specify the lexer to use.
  
          The only difference between this method and the native
          ``setLexer`` method is that expects ``lexer`` to be class, not
          an instance. Another feature is that this method knows which
          ``lexer`` class was installed last, and this information can
          be retrieved with ``qteLexer`` again.
  
          |Args|
  
          * ``lexer`` (**QsciLexer**): lexer class (*not* instance).
  
          |Returns|
  
          **None**
  
          |Raises|
  
          * **QtmacsOtherError** if lexer is not a class.
          """
          if (lexer is not None) and (not issubclass(lexer, Qsci.QsciLexer)):
              QtmacsOtherError('lexer must be a class object and derived from'
                               ' <b>QsciLexer</b>')
              return
  
          # Install and backup the lexer class.
          self.qteLastLexer = lexer
          if lexer is None:
              self.setLexer(None)
          else:
              self.setLexer(lexer())
  
          # Make all fonts in the style mono space.
          self.setMonospace()