If a question of construction becomes material, this necessarily implies that the contract is in every respect valid and enforceable, at least under one of the constructions contended for. Questions as to the validity and enforceability of the contract cannot therefore be involved as a part of a question of construction. They may, of course, be presented in the same case; and a question of construction, when once determined, may also determine the validity of the contract itself. Accordingly, many questions of construction have already been anticipated in connection with the formation of the contract. Still questions of construction are easy to separate from questions of formation, until we reach the question of what terms of the negotiations constitute the terms of the contract. The line of demarkation between this subject and construction is an arbitrary one.1 Construction is in reality a part of the contract. The division is solely for necessary convenience in discussion. When we attempt to distinguish questions of construction from those of the operation of the contract, or from those arising out of discharge, the difficulty of making any logical separation of topics is even greater. Operation and discharge are both dependent on the construction of the contract, if there is any dispute as to its meaning. Accordingly many questions of construction are necessarily left for discussion in connection with, discharge.