A disturbed equilibrium

You will follow a reaction $\text{A} \rightleftharpoons \text{B}$ as it is disturbed.

The idea. $K_c = \dfrac{[\text{B}]}{[\text{A}]}$. If $x$ mol/L of A turns into B on the way to equilibrium, then $\dfrac{b + x}{a - x} = K$, which gives $x = \dfrac{Ka - b}{1 + K}$. A negative $x$ means B turns into A.

Your tasks.

1. In shift, return $x$ for a mixture a, b and a constant K.
2. In settle, return the equilibrium pair (a, b) reached from the mixture.
3. In after_adding, an equilibrium mixture gets added mol/L more A. Return the new equilibrium pair.