#
# This software is Copyright 2005 by Elsevier Inc.  You may use it
# under the terms of the license at http://perl.plover.com/hop/LICENSE.txt .
#



###
### combine2.1
###

## Chapter 7 section 1

sub combine2 {
  my $op = shift;
  my $r;
  $r = sub {
    my ($s, $t) = @_;
    return unless $s && $t;
    node($op->(head($s), head($t)),
         promise { $r->(tail($s), tail($t)) });
  };        
}