code: def _aggr_mean(inList):
    
    aggrSum = 0
    nonNone = 0
    for elem in inList:
      if elem != SENTINEL_VALUE_FOR_MISSING_DATA:
        aggrSum += elem
        nonNone += 1
    if nonNone != 0:
      return aggrSum / nonNone
    else:
      return None
docs: Returns mean of non-None elements of the list