Assertion to check signal change only once(1->0->1) between 2 events

In reply to mlsxdx:

The (##1 $fell(b)[->1] ##1 b[=1] ##1 !b) intersect e[->1];
says that between b and e there must be at least 1 !b; there can be more.

Tou could also write a separate assertion like
not(b && e);