macOS/BSD sed: '1i text' fails with 'command i expects \ followed by text'; prepend a line portably

On macOS, sed '1i import os' file fails with sed: 1: "1i import os": command i expects \ followed by text. GNU sed accepts the one-line form; BSD sed needs 1i\ followed by a newline and then the text, which is awkward to write inside a script. The portable way to prepend a line, which also avoids quoting trouble, is to use a group instead of sed: (echo 'import os'; sed -e 's/a/b/' in.py) > out.py. This also lets you combine the prepend with other substitutions in one pass. The same GNU/BSD split applies to a and c, and to sed -i (BSD needs -i ''). When a script must run on both, prefer writing to a new file over in-place edits.