docs: def single(self, predicate=None):
          '''The only element (which satisfies a condition).
  
          If the predicate is omitted or is None this query returns the only
          element in the sequence; otherwise, it returns the only element in
          the sequence for which the predicate evaluates to True. Exceptions are
          raised if there is either no such element or more than one such
          element.
  
          Note: This method uses immediate execution.
  
          Args:
              predicate: An optional unary predicate function, the only argument
                  to which is the element. The return value should be True for
                  matching elements, otherwise False.  If the predicate is
                  omitted or None the only element of the source sequence will
                  be returned.
  
          Returns:
              The only element of the sequence if predicate is None, otherwise
              the only element for which the predicate returns True.
  
          Raises:
              ValueError: If the Queryable is closed.
              ValueError: If, when predicate is None the source sequence contains
                  more than one element.
              ValueError: If there are no elements matching the predicate or more
                  then one element matching the predicate.
              TypeError: If the predicate is not callable.
          '''
          if self.closed():
              raise ValueError("Attempt to call single() on a closed Queryable.")
  
          return self._single() if predicate is None else self._single_predicate(predicate)