德國 德國民法 民法債編 債編總論 債法 契約法 聯合國國際商品買賣契約條約 歐洲契約法原則 歐洲民法典 歐體 歐體消費性商品買賣指令 債務不履行 義務之違反 給付不能 給付遲延 保護義務之違反 積極侵害債權 締約上之過失 German civil code BGB Law of obligations Contract law The breach of obligations United nations convention on contracts for the international sale of goods Principles of european contract law Study group on a european civil code The dutch civil code EC directive The violation of obligations Impossibility Delay Defects The positive violation of contractual duty Libiality in precontractual negotiations Culpa in contrahendo