def valueFromDataset(cls, datatype, value, metadata=None, tmap=None):
        """
          Return the value contained by dataset argument, after casting to
          correct type and performing type-specific validation
        """
        if tmap is None:
            tmap = typemap
        obj = cls.fromDataset(datatype, value, metadata=metadata, tmap=tmap)
        return obj.value
Return the value contained by dataset argument, after casting to
          correct type and performing type-specific validation