learner versus student - maybe add learner to the glossary?

[lauracion]

"Learner" is used rather than "student" "to avoid confusion when discussing things with people teaching in formal schools, where most people's understanding of 'student' definitely doesn't include teachers or professors." - but this distinction is opaque to most readers. Adding a footnote for its first appearance in the text and/or adding it to the Glossary would keep this message in the book.