Perhaps I should report what was wrong and how I managed to find a solution. If someone else is interested — Xcompose is a handy tool in everyday work for everyone, not just for linguists, e. g.
<Multi_key> <Multi_key> <b> : "₿" # Bitcoin
<Multi_key> <a> <1> : "Often written my whole sentence."
I moved my keymap with the same name to
~.XCompose from MS Windows and tested it by Leafpad. I failed for three reasons.
Leafpad refuses to accept Xcompose input by default, without warning. Although Leafpad is quite smart, it has an option
X input method, accessible by context menu only, and valid until the window is closed. While Mousepad, Zim, MC, even Fish and Sakura write smootly. Not as Gimp and Libre Office. I do not know why, maybe they are not fully unicode compatible. Non-unicode and pseudo-unicode apps in Windows can not accept Wincompose, too.
My nomadBSD does not accept Xcompose input from a keypad, I still have not found a way to fix this yet.
Non-alphanumeric keys should be named verbatim in nomadBSD, e. g.
<Multi_key> <slash> <0> : "∅" # diemeter, not Ø
<Multi_key> </> <0> : "∅"
as was valid in Wincompose. Verbatim names of those keyboard keys are:
Up, Down, Left, Right (the arrow keys), space, exclam, quotedbl,
numbersign, dollar, percent, ampersand, apostrophe, parenleft,
parenright, asterisk, plus, comma, minus, period, slash, colon,
semicolon, less, equal, greater, at, bracketleft, backslash,
bracketright, asciicircum, underscore, grave, braceleft, bar.
Eventually I have set my desired keymap perfectly, even better than Wincompose was.