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    ?- compress([a,a,a,a,b,c,c,a,a,d,e,e,e,e],X).
    X = [a,b,c,a,d,e]

  DB<1>  my $p; print  grep { $p = $_  if  $_ ne $p }  (a,a,a,a,b,c,c,
+a,a,d,e,e,e,e);
abcade

  DB<2>  my $p; print map { $p ne $_ ? $p = $_ : () } (a,a,a,a,b,c,c,a
+,a,d,e,e,e,e)
abcade

 DB<3> use feature "state"; print map { state $p; $p ne $_ ? $p = $_ :
+ () } (a,a,a,a,b,c,c,a,a,d,e,e,e,e)
abcade

  print  grep {  $_ ne $^PRE }  (a,a,a,a,b,c,c,a,a,d,e,e,e,e);
abcade