Feedback on matrix-multiplication

[jiangfangfangxm]

In reference to version 224734d7be7ff0c4a8adb3a206356ea122112f33 in th "Properties of composition" section ,use algebra of transformations to verify S◦(T+U) = S◦T+S◦U .but introduces the definition of T+U later . So suggest move the definition of "algebra of transformations" before the definition of "composition of transformations"