I thought that "span" would be a good tag to add to this question on Mathematics Stack Exchange, but when I click on "Edit Tags", type "span", click on "span", and click on "Save Tag Edits", I don't see the "span" tag appearing, the way an additional tag usually has when I have added other tags to other posts. I found that I could add other tags to that post, and I had this problem when trying to add the "span" tag to other posts, so there seems to be a problem with the "span" tag in general rather than with the post I wish to edit. (I thought it could relate to the fact that "span" has few letters, but I had no trouble with tag "pde", which has fewer.)
P.S. I don't frequent math.meta.stackexchange.com much; is this question appropriate for this forum, or for meta.stackexchange.com? I'm sorry if this is the wrong forum.
J. W. Tanner