NOTES

   1 &=              =>      subdir
   2 &_              =>      subdir_
   3 &/              =>      subdir/
   4 &CAPS           =>      subdir_CAPS
   5 &lc             =>      subdir/lc
   6
   7 &.= &._ &./ &.CAPS &.lc =>      $(top_srcdir)/subdir
   8 &:= &:_ &:/ &:CAPS &:lc =>      $(abs_top_srcdir)/subdir
   9 & thing thing           =>      like &thing &thing (until EOL)
  10
  11 &&              =>      &
  12
  13 &! spc          disables & *until* EOL
  14
  15 &!STUFF         STUFF is recognised instead of & (beyond EOL)
  16                 STUFF is either all punct or all alphanum (incl _)
  17                 any lwsp after STUFF is discarded too
  18 eg notably
  19  STUFF!&                now & is recognised instead (ie back to normal)
  20  STUFFSTUFF     STUFF
  21
  22 eg
  23  &!@@@          @@@ is recognised instead of &
  24  @@@!&          go back to &