Preconditions assertion
A precondition is an assertion which should be true at the initiation of an operation. For instance, a square root operation can't accept a negative argument so a precondition of this operation is that it's argument be non-negative. Preconditions most often specify restrictions on parameters however they may also specify that other conditions have been established, like a file having been created or a device having been initialized. Generally an operation has no preconditions, meaning that it can be executed under any circumstances.