+sub dereference { [ $_[0]->instantiate ] }
+
+sub fold {
+ my ($left, $right, $rev) = @_;
+
+ my (@left, @right);
+
+ if ($my_tc->check($left)) {
+ $left = $left->flatten;
+
+ if ($my_tc->check($right)) {
+ $right = $right->flatten;
+
+ my ($only_left, $common, $only_right) = $inter->(
+ $left->_mods_cache,
+ $right->_mods_cache,
+ );
+
+ my $has_different_layers;
+ for (@$only_left) {
+ if ($_->type eq 'layer') {
+ $has_different_layers = 1;
+ last;
+ }
+ }
+ unless ($has_different_layers) {
+ for (@$only_right) {
+ if ($_->type eq 'layer') {
+ $has_different_layers = 1;
+ last;
+ }
+ }
+ }
+
+ if (!$has_different_layers and @$common) {
+ my $x = $left->new
+ ->mod(@$only_left)
+ ->body($left->_body);
+ my $y = $left->new
+ ->mod(@$only_right)
+ ->body($right->_body);
+ return $left->new
+ ->mod(@$common)
+ ->body(fold($x, $y, $rev));
+ } else {
+ @right = $right->instantiate;
+ }
+ } else {
+ $_body_tc->assert_valid($right);
+ @right = @$right;
+ }
+
+ @left = $left->instantiate;
+ } else {
+ if ($my_tc->check($right)) {
+ return fold($right, $left, 1);
+ } else {
+ $_body_tc->assert_valid($_) for $left, $right;
+ @left = @$left;
+ @right = @$right;
+ }
+ }
+
+ $rev ? [ @right, @left ] : [ @left, @right ];
+}