diff --git a/Python/ast.c b/Python/ast.c
index 0d4d9ee..14500ee 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -1,19 +1,497 @@
 /*
  * This file includes functions to transform a concrete syntax tree (CST) to
- * an abstract syntax tree (AST).  The main function is PyAST_FromNode().
+ * an abstract syntax tree (AST). The main function is PyAST_FromNode().
  *
  */
 #include "Python.h"
 #include "Python-ast.h"
-#include "grammar.h"
 #include "node.h"
 #include "ast.h"
 #include "token.h"
-#include "parsetok.h"
-#include "graminit.h"
 
 #include <assert.h>
 
+static int validate_stmts(asdl_seq *);
+static int validate_exprs(asdl_seq *, expr_context_ty, int);
+static int validate_nonempty_seq(asdl_seq *, const char *, const char *);
+static int validate_stmt(stmt_ty);
+static int validate_expr(expr_ty, expr_context_ty);
+
+static int
+validate_comprehension(asdl_seq *gens)
+{
+    int i;
+    if (!asdl_seq_LEN(gens)) {
+        PyErr_SetString(PyExc_ValueError, "comprehension with no generators");
+        return 0;
+    }
+    for (i = 0; i < asdl_seq_LEN(gens); i++) {
+        comprehension_ty comp = asdl_seq_GET(gens, i);
+        if (!validate_expr(comp->target, Store) ||
+            !validate_expr(comp->iter, Load) ||
+            !validate_exprs(comp->ifs, Load, 0))
+            return 0;
+    }
+    return 1;
+}
+
+static int
+validate_slice(slice_ty slice)
+{
+    switch (slice->kind) {
+    case Slice_kind:
+        return (!slice->v.Slice.lower || validate_expr(slice->v.Slice.lower, Load)) &&
+            (!slice->v.Slice.upper || validate_expr(slice->v.Slice.upper, Load)) &&
+            (!slice->v.Slice.step || validate_expr(slice->v.Slice.step, Load));
+    case ExtSlice_kind: {
+        int i;
+        if (!validate_nonempty_seq(slice->v.ExtSlice.dims, "dims", "ExtSlice"))
+            return 0;
+        for (i = 0; i < asdl_seq_LEN(slice->v.ExtSlice.dims); i++)
+            if (!validate_slice(asdl_seq_GET(slice->v.ExtSlice.dims, i)))
+                return 0;
+        return 1;
+    }
+    case Index_kind:
+        return validate_expr(slice->v.Index.value, Load);
+    default:
+        PyErr_SetString(PyExc_SystemError, "unknown slice node");
+        return 0;
+    }
+}
+
+static int
+validate_keywords(asdl_seq *keywords)
+{
+    int i;
+    for (i = 0; i < asdl_seq_LEN(keywords); i++)
+        if (!validate_expr(((keyword_ty)asdl_seq_GET(keywords, i))->value, Load))
+            return 0;
+    return 1;
+}
+
+static int
+validate_args(asdl_seq *args)
+{
+    int i;
+    for (i = 0; i < asdl_seq_LEN(args); i++) {
+        arg_ty arg = asdl_seq_GET(args, i);
+        if (arg->annotation && !validate_expr(arg->annotation, Load))
+            return 0;
+    }
+    return 1;
+}
+
+static const char *
+expr_context_name(expr_context_ty ctx)
+{
+    switch (ctx) {
+    case Load:
+        return "Load";
+    case Store:
+        return "Store";
+    case Del:
+        return "Del";
+    case AugLoad:
+        return "AugLoad";
+    case AugStore:
+        return "AugStore";
+    case Param:
+        return "Param";
+    default:
+        assert(0);
+        return "(unknown)";
+    }
+}
+
+static int
+validate_arguments(arguments_ty args)
+{
+    if (!validate_args(args->args))
+        return 0;
+    if (args->varargannotation) {
+        if (!args->vararg) {
+            PyErr_SetString(PyExc_ValueError, "varargannotation but no vararg on arguments");
+            return 0;
+        }
+        if (!validate_expr(args->varargannotation, Load))
+            return 0;
+    }
+    if (!validate_args(args->kwonlyargs))
+        return 0;
+    if (args->kwargannotation) {
+        if (!args->kwarg) {
+            PyErr_SetString(PyExc_ValueError, "kwargannotation but no kwarg on arguments");
+            return 0;
+        }
+        if (!validate_expr(args->kwargannotation, Load))
+            return 0;
+    }
+    if (asdl_seq_LEN(args->defaults) > asdl_seq_LEN(args->args)) {
+        PyErr_SetString(PyExc_ValueError, "more positional defaults than args on arguments");
+        return 0;
+    }
+    if (asdl_seq_LEN(args->kw_defaults) != asdl_seq_LEN(args->kwonlyargs)) {
+        PyErr_SetString(PyExc_ValueError, "length of kwonlyargs is not the same as "
+                        "kw_defaults on arguments");
+        return 0;
+    }
+    return validate_exprs(args->defaults, Load, 0) && validate_exprs(args->kw_defaults, Load, 1);
+}
+
+static int
+validate_expr(expr_ty exp, expr_context_ty ctx)
+{
+    int check_ctx = 1;
+    expr_context_ty actual_ctx;
+
+    /* First check expression context. */
+    switch (exp->kind) {
+    case Attribute_kind:
+        actual_ctx = exp->v.Attribute.ctx;
+        break;
+    case Subscript_kind:
+        actual_ctx = exp->v.Subscript.ctx;
+        break;
+    case Starred_kind:
+        actual_ctx = exp->v.Starred.ctx;
+        break;
+    case Name_kind:
+        actual_ctx = exp->v.Name.ctx;
+        break;
+    case List_kind:
+        actual_ctx = exp->v.List.ctx;
+        break;
+    case Tuple_kind:
+        actual_ctx = exp->v.Tuple.ctx;
+        break;
+    default:
+        if (ctx != Load) {
+            PyErr_Format(PyExc_ValueError, "expression which can't be "
+                         "assigned to in %s context", expr_context_name(ctx));
+            return 0;
+        }
+        check_ctx = 0;
+    }
+    if (check_ctx && actual_ctx != ctx) {
+        PyErr_Format(PyExc_ValueError, "expression must have %s context but has %s instead",
+                     expr_context_name(ctx), expr_context_name(actual_ctx));
+        return 0;
+    }
+
+    /* Now validate expression. */
+    switch (exp->kind) {
+    case BoolOp_kind:
+        if (asdl_seq_LEN(exp->v.BoolOp.values) < 2) {
+            PyErr_SetString(PyExc_ValueError, "BoolOp with less than 2 values");
+            return 0;
+        }
+        return validate_exprs(exp->v.BoolOp.values, Load, 0);
+    case BinOp_kind:
+        return validate_expr(exp->v.BinOp.left, Load) &&
+            validate_expr(exp->v.BinOp.right, Load);
+    case UnaryOp_kind:
+        return validate_expr(exp->v.UnaryOp.operand, Load);
+    case Lambda_kind:
+        return validate_arguments(exp->v.Lambda.args) &&
+            validate_expr(exp->v.Lambda.body, Load);
+    case IfExp_kind:
+        return validate_expr(exp->v.IfExp.test, Load) &&
+            validate_expr(exp->v.IfExp.body, Load) &&
+            validate_expr(exp->v.IfExp.orelse, Load);
+    case Dict_kind:
+        if (asdl_seq_LEN(exp->v.Dict.keys) != asdl_seq_LEN(exp->v.Dict.values)) {
+            PyErr_SetString(PyExc_ValueError,
+                            "Dict doesn't have the same number of keys as values");
+            return 0;
+        }
+        return validate_exprs(exp->v.Dict.keys, Load, 0) &&
+            validate_exprs(exp->v.Dict.values, Load, 0);
+    case Set_kind:
+        return validate_exprs(exp->v.Set.elts, Load, 0);
+#define COMP(NAME) \
+        case NAME ## _kind: \
+            return validate_comprehension(exp->v.NAME.generators) && \
+                validate_expr(exp->v.NAME.elt, Load);
+    COMP(ListComp)
+    COMP(SetComp)
+    COMP(GeneratorExp)
+#undef COMP
+    case DictComp_kind:
+        return validate_comprehension(exp->v.DictComp.generators) &&
+            validate_expr(exp->v.DictComp.key, Load) &&
+            validate_expr(exp->v.DictComp.value, Load);
+    case Yield_kind:
+        return !exp->v.Yield.value || validate_expr(exp->v.Yield.value, Load);
+    case Compare_kind:
+        if (!asdl_seq_LEN(exp->v.Compare.comparators)) {
+            PyErr_SetString(PyExc_ValueError, "Compare with no comparators");
+            return 0;
+        }
+        if (asdl_seq_LEN(exp->v.Compare.comparators) !=
+            asdl_seq_LEN(exp->v.Compare.ops)) {
+            PyErr_SetString(PyExc_ValueError, "Compare has a different number "
+                            "of comparators and operands");
+            return 0;
+        }
+        return validate_exprs(exp->v.Compare.comparators, Load, 0) &&
+            validate_expr(exp->v.Compare.left, Load);
+    case Call_kind:
+        return validate_expr(exp->v.Call.func, Load) &&
+            validate_exprs(exp->v.Call.args, Load, 0) &&
+            validate_keywords(exp->v.Call.keywords) &&
+            (!exp->v.Call.starargs || validate_expr(exp->v.Call.starargs, Load)) &&
+            (!exp->v.Call.kwargs || validate_expr(exp->v.Call.kwargs, Load));
+    case Num_kind: {
+        PyObject *n = exp->v.Num.n;
+        if (!PyLong_CheckExact(n) && !PyFloat_CheckExact(n) &&
+            !PyComplex_CheckExact(n)) {
+            PyErr_SetString(PyExc_TypeError, "non-numeric type in Num");
+            return 0;
+        }
+        return 1;
+    }
+    case Str_kind: {
+        PyObject *s = exp->v.Str.s;
+        if (!PyUnicode_CheckExact(s)) {
+            PyErr_SetString(PyExc_TypeError, "non-string type in Str");
+            return 0;
+        }
+        return 1;
+    }
+    case Bytes_kind: {
+        PyObject *b = exp->v.Bytes.s;
+        if (!PyBytes_CheckExact(b)) {
+            PyErr_SetString(PyExc_TypeError, "non-bytes type in Bytes");
+            return 0;
+        }
+        return 1;
+    }
+    case Attribute_kind:
+        return validate_expr(exp->v.Attribute.value, Load);
+    case Subscript_kind:
+        return validate_slice(exp->v.Subscript.slice) &&
+            validate_expr(exp->v.Subscript.value, Load);
+    case Starred_kind:
+        return validate_expr(exp->v.Starred.value, ctx);
+    case List_kind:
+        return validate_exprs(exp->v.List.elts, ctx, 0);
+    case Tuple_kind:
+        return validate_exprs(exp->v.Tuple.elts, ctx, 0);
+    /* These last cases don't have any checking. */
+    case Name_kind:
+    case Ellipsis_kind:
+        return 1;
+    default:
+        PyErr_SetString(PyExc_SystemError, "unexpected expression");
+        return 0;
+    }
+}
+
+static int
+validate_nonempty_seq(asdl_seq *seq, const char *what, const char *owner)
+{
+    if (asdl_seq_LEN(seq))
+        return 1;
+    PyErr_Format(PyExc_ValueError, "empty %s on %s", what, owner);
+    return 0;
+}
+
+static int
+validate_assignlist(asdl_seq *targets, expr_context_ty ctx)
+{
+    return validate_nonempty_seq(targets, "targets", ctx == Del ? "Delete" : "Assign") &&
+        validate_exprs(targets, ctx, 0);
+}
+
+static int
+validate_body(asdl_seq *body, const char *owner)
+{
+    return validate_nonempty_seq(body, "body", owner) && validate_stmts(body);
+}
+
+static int
+validate_stmt(stmt_ty stmt)
+{
+    int i;
+    switch (stmt->kind) {
+    case FunctionDef_kind:
+        return validate_body(stmt->v.FunctionDef.body, "FunctionDef") &&
+            validate_arguments(stmt->v.FunctionDef.args) &&
+            validate_exprs(stmt->v.FunctionDef.decorator_list, Load, 0) &&
+            (!stmt->v.FunctionDef.returns ||
+             validate_expr(stmt->v.FunctionDef.returns, Load));
+    case ClassDef_kind:
+        return validate_body(stmt->v.ClassDef.body, "ClassDef") &&
+            validate_exprs(stmt->v.ClassDef.bases, Load, 0) &&
+            validate_keywords(stmt->v.ClassDef.keywords) &&
+            validate_exprs(stmt->v.ClassDef.decorator_list, Load, 0) &&
+            (!stmt->v.ClassDef.starargs || validate_expr(stmt->v.ClassDef.starargs, Load)) &&
+            (!stmt->v.ClassDef.kwargs || validate_expr(stmt->v.ClassDef.kwargs, Load));
+    case Return_kind:
+        return !stmt->v.Return.value || validate_expr(stmt->v.Return.value, Load);
+    case Delete_kind:
+        return validate_assignlist(stmt->v.Delete.targets, Del);
+    case Assign_kind:
+        return validate_assignlist(stmt->v.Assign.targets, Store) &&
+            validate_expr(stmt->v.Assign.value, Load);
+    case AugAssign_kind:
+        return validate_expr(stmt->v.AugAssign.target, Store) &&
+            validate_expr(stmt->v.AugAssign.value, Load);
+    case For_kind:
+        return validate_expr(stmt->v.For.target, Store) &&
+            validate_expr(stmt->v.For.iter, Load) &&
+            validate_body(stmt->v.For.body, "For") &&
+            validate_stmts(stmt->v.For.orelse);
+    case While_kind:
+        return validate_expr(stmt->v.While.test, Load) &&
+            validate_body(stmt->v.While.body, "While") &&
+            validate_stmts(stmt->v.While.orelse);
+    case If_kind:
+        return validate_expr(stmt->v.If.test, Load) &&
+            validate_body(stmt->v.If.body, "If") &&
+            validate_stmts(stmt->v.If.orelse);
+    case With_kind:
+        if (!validate_nonempty_seq(stmt->v.With.items, "items", "With"))
+            return 0;
+        for (i = 0; i < asdl_seq_LEN(stmt->v.With.items); i++) {
+            withitem_ty item = asdl_seq_GET(stmt->v.With.items, i);
+            if (!validate_expr(item->context_expr, Load) ||
+                (item->optional_vars && !validate_expr(item->optional_vars, Store)))
+                return 0;
+        }
+        return validate_body(stmt->v.With.body, "With");
+    case Raise_kind:
+        if (stmt->v.Raise.exc) {
+            return validate_expr(stmt->v.Raise.exc, Load) &&
+                (!stmt->v.Raise.cause || validate_expr(stmt->v.Raise.cause, Load));
+        }
+        if (stmt->v.Raise.cause) {
+            PyErr_SetString(PyExc_ValueError, "Raise with cause but no exception");
+            return 0;
+        }
+        return 1;
+    case Try_kind:
+        if (!validate_body(stmt->v.Try.body, "Try"))
+            return 0;
+        if (!asdl_seq_LEN(stmt->v.Try.handlers) &&
+            !asdl_seq_LEN(stmt->v.Try.finalbody)) {
+            PyErr_SetString(PyExc_ValueError, "Try has neither except handlers nor finalbody");
+            return 0;
+        }
+        if (!asdl_seq_LEN(stmt->v.Try.handlers) &&
+            asdl_seq_LEN(stmt->v.Try.orelse)) {
+            PyErr_SetString(PyExc_ValueError, "Try has orelse but no except handlers");
+            return 0;
+        }
+        for (i = 0; i < asdl_seq_LEN(stmt->v.Try.handlers); i++) {
+            excepthandler_ty handler = asdl_seq_GET(stmt->v.Try.handlers, i);
+            if ((handler->v.ExceptHandler.type &&
+                 !validate_expr(handler->v.ExceptHandler.type, Load)) ||
+                !validate_body(handler->v.ExceptHandler.body, "ExceptHandler"))
+                return 0;
+        }
+        return (!asdl_seq_LEN(stmt->v.Try.finalbody) ||
+                validate_stmts(stmt->v.Try.finalbody)) &&
+            (!asdl_seq_LEN(stmt->v.Try.orelse) ||
+             validate_stmts(stmt->v.Try.orelse));
+    case Assert_kind:
+        return validate_expr(stmt->v.Assert.test, Load) &&
+            (!stmt->v.Assert.msg || validate_expr(stmt->v.Assert.msg, Load));
+    case Import_kind:
+        return validate_nonempty_seq(stmt->v.Import.names, "names", "Import");
+    case ImportFrom_kind:
+        if (stmt->v.ImportFrom.level < -1) {
+            PyErr_SetString(PyExc_ValueError, "ImportFrom level less than -1");
+            return 0;
+        }
+        return validate_nonempty_seq(stmt->v.ImportFrom.names, "names", "ImportFrom");
+    case Global_kind:
+        return validate_nonempty_seq(stmt->v.Global.names, "names", "Global");
+    case Nonlocal_kind:
+        return validate_nonempty_seq(stmt->v.Nonlocal.names, "names", "Nonlocal");
+    case Expr_kind:
+        return validate_expr(stmt->v.Expr.value, Load);
+    case Pass_kind:
+    case Break_kind:
+    case Continue_kind:
+        return 1;
+    default:
+        PyErr_SetString(PyExc_SystemError, "unexpected statement");
+        return 0;
+    }
+}
+
+static int
+validate_stmts(asdl_seq *seq)
+{
+    int i;
+    for (i = 0; i < asdl_seq_LEN(seq); i++) {
+        stmt_ty stmt = asdl_seq_GET(seq, i);
+        if (stmt) {
+            if (!validate_stmt(stmt))
+                return 0;
+        }
+        else {
+            PyErr_SetString(PyExc_ValueError,
+                            "None disallowed in statement list");
+            return 0;
+        }
+    }
+    return 1;
+}
+
+static int
+validate_exprs(asdl_seq *exprs, expr_context_ty ctx, int null_ok)
+{
+    int i;
+    for (i = 0; i < asdl_seq_LEN(exprs); i++) {
+        expr_ty expr = asdl_seq_GET(exprs, i);
+        if (expr) {
+            if (!validate_expr(expr, ctx))
+                return 0;
+        }
+        else if (!null_ok) {
+            PyErr_SetString(PyExc_ValueError,
+                            "None disallowed in expression list");
+            return 0;
+        }
+            
+    }
+    return 1;
+}
+
+int
+PyAST_Validate(mod_ty mod)
+{
+    int res = 0;
+
+    switch (mod->kind) {
+    case Module_kind:
+        res = validate_stmts(mod->v.Module.body);
+        break;
+    case Interactive_kind:
+        res = validate_stmts(mod->v.Interactive.body);
+        break;
+    case Expression_kind:
+        res = validate_expr(mod->v.Expression.body, Load);
+        break;
+    case Suite_kind:
+        PyErr_SetString(PyExc_ValueError, "Suite is not valid in the CPython compiler");
+        break;
+    default:
+        PyErr_SetString(PyExc_SystemError, "impossible module node");
+        res = 0;
+        break;
+    }
+    return res;
+}
+
+/* This is down here, so defines like "test" don't intefere with access AST above. */
+#include "grammar.h"
+#include "parsetok.h"
+#include "graminit.h"
+
 /* Data structure used internally */
 struct compiling {
     char *c_encoding; /* source encoding */