or L{None}, asks the current sentence for the C{unitKey}. If that
            also fails, raises C{AttributeError}.
        @type unit: C{str}
        @param unitKey: The name of the key/attribute under which the unit can
            be found in the current sentence. If the C{unit} parameter is set,
            this parameter is not used.
        @type unitKey: C{str}
        @param sourceKey: The name of the key/attribute that contains the
            current value to be converted (expressed in units as defined
            according to the C{unit} parameter). If unset, will use the
            same key as the value key.
        @type sourceKey: C{str}
        @param valueKey: The key name in which the data will be stored in the
            C{_sentenceData} instance attribute. If unset, attempts to remove
            "Units" from the end of the C{unitKey} parameter. If that fails,
            raises C{ValueError}.
        @type valueKey: C{str}
        NZ