# # 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)) }); }; }