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We add the equation $\pi(a)+\pi(b)=1$ because $\pi$ is a valid probability distribution, meaning its elements must sum to 1. This provides the second independent equation needed to solve the underdetermined system created by the redundant balance equations.

Conditions: $\pi$ is a probability distribution over the states.; The state space has exactly two states, a and b.

We add the equation $\pi(a)+\pi(b)=1$ because $\pi$ is a valid probability distribution, meaning its elements must sum to 1. This provides the second independent equation needed to solve the underdetermined system created by the redundant balance equations.

Conditions: $\pi$ is a probability distribution over the states.; The state space has exactly two states, a and b.