1. In the first case, the previous code was failing during type inference
due to mismatched structure. Fix is to use the X structure at both
points in the code.
2. In the second case, a naive transcription that subsitutes *nothing*
in for the omitted statements signified by "..." will actually
compile without an error. Furthermore, any pure code could also be
substituted for the ellipsis and the code would compile (as the
text already states). So to make the example more illustrative, it
would be better to include an impure callback, which makes the
potential for aliasing immediately obvious to the reader.
