25.17.6 assert

The assert statement allows you test if a condition is true, if not, the program will raise an exception with code EC_assert.

assert (condition)
assert (condition, message)

The optional message will be part of the msg field of raised Exception to explain the situation. If no message is explicitly specified, msg is set to the source code of the expression evaluated as the assert condition.

assert (1 == 1);
assert (0 == 0, "Zero is equal to zero");

assert is useful for writing unit tests.