DB<106> sub friends (&@){
            my $code=shift;
            $last=$_;
            my @result;
            for (@_) {
              push @result, $code->($last,$_);
              $last=$_;
            }
            return @result;
          }

  DB<107> friends { $_[0]+$_[1] } 1..5
 => (1, 3, 5, 7, 9)