tree: support indexing and range extraction.

* lib.c (do_generic_funcall): Support tree object invocation
with one or two arguments via sub and ref.
(sub): Implement for trees via sub_tree.
(ref): Implement for trees via tree_lookup.

* tree.c (sub_tree): New function.
(tree_init): Register sub-tree intrinsic.

* tree.h (sub_tree): Declared.

* tests/010/tree.tl: New tests.

* txr.1: Documented: DWIM bracket syntax on trees, sub and ref
support for trees, sub-tree function,

* share/txr/stdlib/doc-syms.tl: Regenerated.

1 files changed, 1 insertions, 0 deletions

diff --git a/tree.h b/tree.h
index 7b045f1a..072bc6df 100644
--- a/tree.h
+++ b/tree.h
@@ -57,4 +57,5 @@ val tree_reset_at(val iter, val tree, val lowkey);
 val tree_next(val iter);
 val tree_peek(val iter);
 val tree_clear(val tree);
+val sub_tree(val tree, val from, val to);
 void tree_init(void);