An assert is a command encoding the programmer's opinion that a particular condition must hold at a particular point in the program. They (should) have no effect on the state of the program when the condition is fulfilled, but may throw an error (or the like) when the condition fails; this helps to bring bugs breaking the condition to the attention of the programmer.

General implementations of assertions:

One particular assertion idiom is the use of incr to ensure a variable holds an integer value:

 incr number 0