def last_name=(last_name)
      name = Name.new('First', last_name)
      raise "invalid last name" unless name.last.length > 0 && name.first.length > 0
      @last_name = name.last
    end
def last_name=(last_name)
      name = Name.new('First', last_name)
      raise "invalid last name" unless name.last.length > 0 && name.first.length > 0
      @last_name = name.last
    end
Canonicalise and set the last name(s).