"The incompleteness of property rights in general creates well-known problems in welfare economics, being in fact the basic component of externalities. In particular, markets for future commitments are relatively under-developed compared with those for the present or immediate future. Individuals have to supply for themselves expectations as to future developments in order to make decisions with consequences extending into the future, e.g., investments. These expectations, for example of prices or of supply availabilities, are not "property," but they influence the use of property and are taken into account in the present legal system. For example, an obligation to sell a product for the next few years at a given price is understood in the law to hold only if conditions do not change in a strongly unexpected way; this understanding does not require explicit statement."