Caller will search the stack, looking for a Terst testing scope.

Is compares two values (got & expect) and returns true if the comparison is true, false otherwise.

IsErr compares two values (got & expect) and returns nil if the comparison is true, an ErrFail if the comparison is false, or an ErrInvalid if the comparison is invalid.

Terst creates a testing scope, where Is can be called and errors will be reported according to the top-level location of the comparison, and not where the Is call actually takes place.

Call is a reference to a line immediately under a Terst testing scope.