Most systems also have flags or switches that allow this checking to be turned off once confidence in the program has been established. When carefully chosen, the tests can also be used to prove that a program is correct . To do this it is necessary to prove that the program terminates and that it produces the correct results when it terminates. The tests that can be included in a program are boolean expressions. The Abstract Data Type Boolean.
Operators vary between programming languages.
Note that these rules are mathematical ideals for a language to attempt to implement. Some languages, such as Pascal, do not take these short-cuts. Sometimes to prove a program it is necessary to use logical expressions that are not easily expressed as boolean expressions in the programming language. In such cases comments can be used. They are not checked automatically but they make very informative comments for another reader.