  << ^ N I - I N C R E M E N T   R E T U R N   L I N K >>

 ^NI can be used to make a macro subroutine into a conditional one or even a
many-way switch, or simply for a called macro subroutine to skip a section of
the calling macro. It increases the link that ^ND stacked by 2 characters, i.e.
after one ^NI the called macro will return 2 characters further on than
otherwise.
 A check is made that after ^NI in the called macro there are at least 2
characters (so macro can do a ^NU). Then, if the macro has not actually been
called by another macro, ^NI is ignored otherwise a check is made that the
macro still has somewhere to return to (i.e. the link has not been pushed
beyond returning just to the end of the macro). Failure of any check results in
error action: the bell rings, macro mode is quitted, and stack is emptied.
