In GNU Emacs, you're always in "insert mode"; i.e., what you type will
be inserted into your file at the current cursor position, pushing
along whatever text follows. The RET key is just like any other key; it
causes a RET (or newline) to be placed in the file. If you press RET in
the middle of a line, that line will be split into two. In addition, to
join two lines together, just delete the newline between them.
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