The form formula_138 is "the" composition of formula_139 and formula_140. We see that its first coefficient is well-defined, but the other two depend on the choice of "B" and "C". One way to make this a well-defined operation is to make an arbitrary convention for how to choose "B"—for instance, choose "B" to be the smallest positive solution to the system of congruences above. Alternatively, we may view the result of composition, not as a form, but as an equivalence class of forms modulo the action of the group of matrices of the form