TreeNode<Decl<V>> pattern() {
		return _pattern;
	}
Create a new tree-pattern object from the given pattern tree.

@param pattern the pattern-tree
@throws NullPointerException if the given {@code pattern} is {@code null}
@throws IllegalArgumentException if {@link Var} nodes are not leaf nodes;
        {@link Tree#isLeaf()} is {@code false}
jenetics.ext/src/main/java/io/jenetics/ext/rewriting/TreePattern.java