first_couple  {$a != $b}  0 ..9
any_couple ...
map_couple ...
...

##</code><code>##

map_ante { $_ == $^PRE ? $_ : () }
grep_ante
...

##</code><code>##

xgrep { $_ == 9 }     LIST  # like ordinary grep 
xgrep { $a == $b }    LIST  # like grep_couple
xgrep { $^PRE == $_ } LIST  # like grep_ante
xgrep { $a == $^PRE } LIST  # ERROR
 
xfirst ...                  # and so on