In this video (https://youtu.be/GzBGa1BCXgs?t=267), it is said that the inner function in a composition must be a scalar and can't be a vector. What does this mean? It is common in multivariable calculus to consider functions with multiple inputs, which can be viewed geometrically as a single point or vector. Here (https://youtu.be/qZlBjnC3iro?t=162) is an example of a vector being plugged into a function, in fact. In what sense does plugging a vector into a function not work?