diff -r ac381dd7a21f -r 3f1a9b44b8c2 mercurial/parser.py
--- a/mercurial/parser.py	Sun Apr 26 19:50:42 2015 +0900
+++ b/mercurial/parser.py	Sun Apr 26 22:20:03 2015 +0900
@@ -93,3 +93,18 @@
         if self._methods:
             return self.eval(t)
         return t
+
+def prettyformat(tree, leafnodes):
+    def _prettyformat(tree, level, lines):
+        if not isinstance(tree, tuple) or tree[0] in leafnodes:
+            lines.append((level, str(tree)))
+        else:
+            lines.append((level, '(%s' % tree[0]))
+            for s in tree[1:]:
+                _prettyformat(s, level + 1, lines)
+            lines[-1:] = [(lines[-1][0], lines[-1][1] + ')')]
+
+    lines = []
+    _prettyformat(tree, 0, lines)
+    output = '\n'.join(('  ' * l + s) for l, s in lines)
+    return output