Squashed commit of the following:

commit 0002a250da9b4f67080d2024512774a539998272
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Sat Nov 12 20:41:31 2022 +0000

    Refactor to remove codeop/codeatom/cell tags and replace with core HTML spans.

commit a42f1d4aad785361b6fcff61e046518fc8f5ecaa
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Sat Nov 12 08:12:09 2022 +0000

    Code tidy.

commit 1bd47da9b318bb66334ffb9a8c68df7ff6c2d354
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Sat Nov 12 07:44:55 2022 +0000

    Code tidy.

commit 27ae606d6c26569a1cc1270d859491df163b88d7
Merge: 4806b989 e4d7432e
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Sat Nov 12 05:42:53 2022 +0000

    Merge remote-tracking branch 'origin/dev' into trees

commit 4806b9890dc8a5ba5b4943114778c297094cbe8e
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Sat Nov 12 05:42:26 2022 +0000

    Release candidate for tree functionality.

commit 76d3e9907746987a73fd2b26ab6624bf30b61672
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Sat Nov 12 00:13:36 2022 +0000

    Fix to issue #878.

commit c1b839182a2091fa6b05ffd8f7f6c9cdea7a7f93
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Thu Nov 10 19:30:12 2022 +0000

    Change causes failure in unit tests, illustrating issue #878.

commit 835a2b5c06b806ad4a2ecf4c02a0465c0147485d
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Thu Nov 10 19:25:56 2022 +0000

    WIP: improve style and expand range of operators faithfully displayed.

commit 60d4f4c22a56c3e2f6022b8a3e660e6f9037da98
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Wed Nov 9 08:48:31 2022 +0000

    Remove figure and update styling.

commit ec0afb0eb0892f49ffa54defc81b03598685ef7a
Author: Chris Sangwin <C.J.Sangwin@ed.ac.uk>
Date:   Mon Nov 7 15:01:02 2022 +0000

    Fix typo in docs.

commit b5143b81032c61e2f0fe09b1b6511937949ac092
Author: Chris Sangwin <csangwin@stack.local>
Date:   Mon Nov 7 08:11:39 2022 +0000

    WIP: first attempt at using the tree CSS.