1. A method for creating a plurality of output test vectors from a statechart, wherein the statechart specifies requirements of a system function to be executed by a system-performing device and comprises a plurality of states, a plurality of variables and a plurality of transitions, wherein at least one transition of the plurality of transitions comprises a condition expressed in terms of at least one variable in the plurality of variables, comprising: providing a processor, and data storage having machine-language instructions executable by the processor to perform a method comprising: generating a plurality of forward-reached-transition environments using a forward-propagation pass through the statechart, wherein each forward-reached-transition environment in the plurality of forward-reached-transition environments is generated for a forward-reached transition forward propagated during the forward-propagation pass; determining a transition was unreached during the forward-propagation pass; responsive to the determination was unreached during the forward-propagation pass, generating a plurality of backward-reached-transition environments via a backward-propagation pass through the statechart, wherein each backward-reached-transition environment in the plurality of backward-reached-transition environments is generated for a backward-reached transition propagated during the backward-propagation pass; determining a transition was unreached during both the forward-propagation pass and the backward-propagation pass; responsive to the determination that a transition was unreached during both the forward-propagation and backward-propagation pass, performing alternating passes of forward-propagation and backwards-propagation until an insufficient number of transitions has been reached by a pass of forward-propagation and backward-propagation; generating a plurality of output test vectors from the plurality of forward-reached-transition environments and the plurality of backward-reached-transition environments; and outputting the plurality of output test vectors.