sentences:  So if A promises to give B a book and B accepts the offer without giving anything in return, B would have a legal right to the book and A could not change her mind about giving it to B as a gift.However, in common law systems the concept of "culpa in contrahendo", a form of estoppel, is increasingly used to create obligations during pre-contractual negotiations.Estoppel is an equitable doctrine that provides for the creation of legal obligations if a party has given another an assurance and the other has relied on the assurance to his detriment.