Thanks Simon, and everyone else who responded.
I saw that in the FAQ about the possibility of time-limited editing of posts, but I couldn't figure out why that would be used, so I thought it might be something else. I still don't know why we can only edit our posts for a while (how long?) but if those are the rules, I'll just have to remember it and read my posts really carefully right away (I sometimes find typos later on that I like to fix, but...)
I have always wanted to be able to use Jim as my username on a forum, so I always try it first, but this is the first time I actually got to use it. I guess everyone else assumed their names were taken and so didn't bother trying (at least everyone else named Jim!

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Jim
James V. Signorile, ASCAP