external procedure treeinitforest(var f: treeforest);
external function treecreate(e: treelement; var f: treeforest): tree;
external function treempty(f: treeforest): tree;
external function treebuild(e: treelement; l,r: tree; var f:treeforest): tree;
external function treeleft(t: tree; f: treeforest): tree;
external function treeright(t: tree; f: treeforest): tree;
external function treerootelem(t: tree; f: treeforest): treelement;
external procedure treeinorder(t: tree; f: treeforest; var out: text);

.