assert - program verification
Assert is a macro that indicates expression is expected to be true at
this point in the program. It causes an exit(2) with a diagnostic com-
ment on the standard output when expression is false (0). Compiling
with the cc(1) option -DNDEBUG effectively deletes assert from the pro-
`Assertion failed: file f line n.' F is the source file and n the
source line number of the assert statement.