&=		=>	subdir
&_		=>	subdir_
&/		=>	subdir/
&CAPS		=>	subdir_CAPS
&lc		=>	subdir/lc

&.= &._ &./ &.CAPS &.lc	=>	$(top_srcdir)/subdir
&:= &:_ &:/ &:CAPS &:lc	=>	$(abs_top_srcdir)/subdir
& thing thing		=>	like &thing &thing (until EOL)

&&		=>	&

&! spc		disables & *until* EOL

&!STUFF		STUFF is recognised instead of & (beyond EOL)
		STUFF is either all punct or all alphanum (incl _)
		any lwsp after STUFF is discarded too
eg notably
 STUFF!&		now & is recognised instead (ie back to normal)
 STUFFSTUFF	STUFF

eg
 &!@@@		@@@ is recognised instead of &
 @@@!&		go back to &