code: def add_field(self, field_instance_or_string):
          
          if isinstance(field_instance_or_string, basestring):
              field_instance = Field(field_instance_or_string)
          elif isinstance(field_instance_or_string, Field):
              field_instance_or_string = field_instance
          else:
              raise ValueError('Expected a basetring or Field instance')
  
          self.fields.append(field_instance)
  
          return self
docs: Appends a field, can be a :class:`~es_fluent.fields.Field` or string.