Re-add the _ast docs to 3k; rename it accordingly.

Doc-26/library/_ast.rst

@@ -4,6 +4,7 @@ Abstract Syntax Trees
 =====================
 
 .. module:: _ast
+   :synopsis: Abstract Syntax Tree classes.
 
 .. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>
 

Doc-26/library/compiler.rst

@@ -30,15 +30,16 @@ work.  It blends reference material with a tutorial.
 
 The following modules are part of the :mod:`compiler` package:
 
-
 .. toctree::
 
-   ast.rst
+   _ast.rst
+
 
 The basic interface
 ===================
 
 .. module:: compiler
+   :synopsis: Python code compiler written in Python.
 
 
 The top-level of the package defines four functions.  If you import

Doc-3k/library/_ast.rst

@@ -0,0 +1,59 @@
+.. _ast:
+
+Abstract Syntax Trees
+=====================
+
+.. module:: _ast
+   :synopsis: Abstract Syntax Tree classes.
+
+.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>
+
+
+.. versionadded:: 2.5
+
+The ``_ast`` module helps Python applications to process trees of the Python
+abstract syntax grammar. The Python compiler currently provides read-only access
+to such trees, meaning that applications can only create a tree for a given
+piece of Python source code; generating byte code from a (potentially modified)
+tree is not supported. The abstract syntax itself might change with each Python
+release; this module helps to find out programmatically what the current grammar
+looks like.
+
+An abstract syntax tree can be generated by passing ``_ast.PyCF_ONLY_AST`` as a
+flag to the :func:`compile` builtin function. The result will be a tree of
+objects whose classes all inherit from ``_ast.AST``.
+
+The actual classes are derived from the ``Parser/Python.asdl`` file, which is
+reproduced below. There is one class defined for each left-hand side symbol in
+the abstract grammar (for example, ``_ast.stmt`` or ``_ast.expr``). In addition,
+there is one class defined for each constructor on the right-hand side; these
+classes inherit from the classes for the left-hand side trees. For example,
+``_ast.BinOp`` inherits from ``_ast.expr``. For production rules with
+alternatives (aka "sums"), the left-hand side class is abstract: only instances
+of specific constructor nodes are ever created.
+
+Each concrete class has an attribute ``_fields`` which gives the names of all
+child nodes.
+
+Each instance of a concrete class has one attribute for each child node, of the
+type as defined in the grammar. For example, ``_ast.BinOp`` instances have an
+attribute ``left`` of type ``_ast.expr``.   Instances of ``_ast.expr`` and
+``_ast.stmt`` subclasses also have lineno and col_offset attributes.  The lineno
+is the line number of source text (1 indexed so the first line is line 1) and
+the col_offset is the utf8 byte offset of the first token that generated the
+node.  The utf8 offset is recorded because the parser uses utf8  internally.
+
+If these attributes are marked as optional in the grammar (using a question
+mark), the value might be ``None``. If the attributes can have zero-or-more
+values (marked with an asterisk), the values are represented as Python lists.
+
+
+Abstract Grammar
+----------------
+
+The module defines a string constant ``__version__`` which is the decimal
+subversion revision number of the file shown below.
+
+The abstract grammar is currently defined as follows:
+
+.. literalinclude:: ../../Parser/Python.asdl

Doc-3k/library/language.rst

@@ -15,6 +15,7 @@ These modules include:
 .. toctree::
 
    parser.rst
+   _ast.rst
    symbol.rst
    token.rst
    keyword.rst