The above equation can be interpreted as follows. The initial sum on the RHS is over all reactions. For each reaction j , the brackets immediately following the sum give two terms. The term with the simple coefficient −1 gives the probability flux away from a given state X due to reaction j changing the state. The term preceded by the product of step operators gives the probability flux due to reaction j changing a different state X ′ into state X . The product of step operators constructs this state X ′ .