docs: def validate(self, document):
          '''Check if the selected template exists.'''
  
          template = document.text
  
          if template not in self.builtin_templates:
              raise ValidationError(
                  message=f'Template {template} not found. '
                  + f'Available templates are: {", ".join(self.builtin_templates)}.',
                  cursor_position=0
              )