NOTES

   1 &CAPS           =>      subdir_CAPS                     or TOP_CAPS
   2 &lc             =>      subdir/lc                       or lc
   3
   4 &_              =>      subdir_                         or TOP_
   5 &/              =>      subdir/                         or nothing
   6 &=_             =>      subdir                          or TOP
   7 &=/             =>      subdir                          or .
   8 &^              =>      $(top_srcdir)/subdir            or $(top_srcdir)
   9 &~              =>      $(abs_top_srcdir)/subdir        or $(abs_top_srcdir)
  10
  11 &&              =>      &&
  12 \&              =>      &
  13
  14 & thing thing... &      =>      each thing prefixed by &/ &^ &~ resp
  15 & ^ thing thing... &            each thing is any non-ws
  16 & ~ thing thing... &            & may be omitted before EOL or before \EOL
  17                                 other &'s not recognised
  18
  19 CAPS is [A-Z][0-9_A-Z]*(?!\w)
  20 lc is [a-z][-+,0-9_a-z]*(?!\w)
  21
  22 &!<spaces or tabs>      disables & *until* EOL (and disappears)
  23
  24 &!STUFF         STUFF is recognised instead of & (beyond EOL)
  25                 STUFF is either all ASCII punct or all ASCII alphanum (incl _)
  26                 any lwsp after STUFF is discarded too
  27
  28 eg notably
  29  STUFF!&                now & is recognised instead (ie back to normal)
  30  STUFFSTUFF     STUFF
  31
  32 eg
  33  &!@@@          @@@ is recognised instead of &
  34  @@@!&          go back to &
  35
  36 &TARGETS[_things]               is handled specially
  37                                 must be spelled precisely this way
  38                                 if no _things, means _all
  39
  40 Also, `all' is weird in that it is present even if not specified