1. An integrated circuit design verification generator comprising program elements residing on a non-transitory computer-readable storage-medium, the program elements enabling the computer to cause the generator to: selectively generate a random verification scenario, selectively receive a verification scenario that is injected from a higher stage level generator, decode the verification scenario and generate a sequence of lower level scenarios using a callback mechanism, where for each verification scenario, a callback is added that can decode the verification scenario and generate the sequence of lower level scenarios, selectively inject the sequence of lower level scenarios into a lower stage level generator, and selectively synchronize the generator at one of a sub-system level and a system level to wait for an interrupt from an external event, check a status of the interrupt, and clear the interrupt after each verification scenario.