Q+= is the q-operator for expressing an additive component of somesimulator variable.
Eg: (Q+= (Battery-1 current) (Term-11 current)) which asserts that
the current of Battery-1 is composed of a sum of which
the current thru Term-11 is a component.
A closed-world-assumption is needed to convert a bunch of such Q+= eqns
into a Q= eqn.