An execution model specifies how work takes place. Every programming language has an execution model, which is specified as part of the language specification, and is implemented as part of the language implementation. Execution models can also exist independently from programming languages, an example of which would be the POSIX Threads library. The details in the specification of an execution model cover things such as what is an indivisible unit of work and what are the constraints on the order in which those units of work take place. For example, the addition operation is an indivisible unit of work in many languages, and in sequential languages such units of work are constrained to take place one after the other.