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Subclass of Num where (*) is commutative.

Num doesn't demand commutative (*), and there are reasonable "real-world" instances with non-commutative multiplication. There is also no canonical subclass in base that would suffice, as both Integral and Floating imply commutative (*) for different reasons.

Two examples of non-commutative (*):

• Linear.Quaternion.Quaterion from the linear package has a Num instance, and quaternion multiplication is noncommutative.
• Data.Matrix.Matrix from the matrix package uses (*) for matrix multiplication, which is also non-commutative (on square matrices, which is the only time the question makes sense).

Since: 0.1.0