docs: def _get_operator_fn(self, op):
          """
          Returns the function in this module that corresponds to an operator string.
          This simply checks if there is a method that handles the operator defined
          in this module, replacing '$' with '_' (i.e. if this module has a _gt
          method for $gt) and returns it. If no match is found, or the operator does not
          start with '$', a MalformedQueryException is raised
          """
          if not op.startswith('$'):
              raise MalformedQueryException("Operator '%s' is not a valid query operation" % op)
  
          try:
              return getattr(sys.modules[__name__], op.replace('$', '_'))
          except AttributeError:
              raise MalformedQueryException("Operator '%s' is not currently implemented" % op)