As we normally use <px;;0> to capture the margin note, and <tr;0;f> to read the text register for print out the margin note, these instance allows once only in an open text line, so adding additional number register to indicate right hand side and left hand side would not work.

In case there is more than one margin notes on the same line (assuming one for left and one for right),  is there any way to settle it as only one frill text register (px;;0) we can use for the margin note?

It would be nice though if !fg returns an integer instead of 0 or 1.

That would take away the need of setting (and don't forget) resetting various number registers.

There is only 2 states for the !flag, 1 or 0.  It's purpose isn't to indicate other states (i.e. no margin note, left and/or right).  However, if you use the </fg> to simply mark the line AND use number registers (NOT x-register!!!!!), your frills processing macros (not recommended - use Perl - MUCH easier) can not only detect the !flag line(s) but also test the number register values.

E.g.  #6 for left margin note indicator and #7 for right. Set #6 to '1' for a left margin note and #7 to '1' for a right margin note. Now, when your code checks for a !flag line, it should also test for #6=1 and create the left margin note and #7=1 to create the right margin note.

If you're short on spare number registers you can 'split' a number register into high and low. In this case you could use #6 high and #6 low.

To set high: <nsh;6;1>

To set low:  <nsl;6;1>

To reset: <nsh;6;0> or <nsl;6;0>

Now you're code only has to test for !flag and fetch #6. In macros you can do this with <nrh;6> and <nrl;6>. In Perl its a little more interesting as you can only fetch the whole register. You then need to do some Perl math to extract the upper and lower 8 bits of the #6 value.  Use modulo (% 256) to extract the low and divide (/ 256) to extract the high.