def unit=(unit_val)
      unless unit_val.is_a?(EEML::Unit)
        raise TypeError.new("unit must be an EEML::Unit") 
      end
      @unit = unit_val
    end
def unit=(unit_val)
      unless unit_val.is_a?(EEML::Unit)
        raise TypeError.new("unit must be an EEML::Unit") 
      end
      @unit = unit_val
    end
Unit of measurement - must be a EEML::Unit object