I would suggest that "affine" would be a better name than
"linear" for the properties of the access functions.  Linear
is OK (as in linear as opposed to quadratic polynomials), but in
this context with all the "vector"s and "array"s, I think affine
would be better.

Also, instead of saying

It is an error to store anything in an element of a shape.

can one just say that "Shapes are immutable"?

Brad Lucier