diff --git a/sphinx/pycode/Grammar-py2.txt b/sphinx/pycode/Grammar-py2.txt
index 54d6fa970..98bd1f22b 100644
--- a/sphinx/pycode/Grammar-py2.txt
+++ b/sphinx/pycode/Grammar-py2.txt
@@ -1,12 +1,15 @@
 # Grammar for Python 2.x
 
+# IMPORTANT: when copying over a new Grammar file, make sure file_input
+# is the first nonterminal in the file!
+
 # Start symbols for the grammar:
 #       single_input is a single interactive statement;
 #       file_input is a module or sequence of commands read from an input file;
 #       eval_input is the input for the eval() and input() functions.
 # NB: compound_stmt in single_input is followed by extra NEWLINE!
-single_input: NEWLINE | simple_stmt | compound_stmt NEWLINE
 file_input: (NEWLINE | stmt)* ENDMARKER
+single_input: NEWLINE | simple_stmt | compound_stmt NEWLINE
 eval_input: testlist NEWLINE* ENDMARKER
 
 decorator: '@' dotted_name [ '(' [arglist] ')' ] NEWLINE
@@ -94,7 +97,6 @@ atom: ('(' [yield_expr|testlist_comp] ')' |
        '[' [listmaker] ']' |
        '{' [dictorsetmaker] '}' |
        '`' testlist1 '`' |
-#       '>>' test |
        NAME | NUMBER | STRING+)
 listmaker: test ( list_for | (',' test)* [','] )
 testlist_comp: test ( comp_for | (',' test)* [','] )
diff --git a/sphinx/pycode/Grammar-py3.txt b/sphinx/pycode/Grammar-py3.txt
index 92922accf..083b5f91e 100644
--- a/sphinx/pycode/Grammar-py3.txt
+++ b/sphinx/pycode/Grammar-py3.txt
@@ -1,12 +1,15 @@
 # Grammar for Python 3.x (with at least x <= 4)
 
+# IMPORTANT: when copying over a new Grammar file, make sure file_input
+# is the first nonterminal in the file!
+
 # Start symbols for the grammar:
 #       single_input is a single interactive statement;
 #       file_input is a module or sequence of commands read from an input file;
 #       eval_input is the input for the eval() functions.
 # NB: compound_stmt in single_input is followed by extra NEWLINE!
-single_input: NEWLINE | simple_stmt | compound_stmt NEWLINE
 file_input: (NEWLINE | stmt)* ENDMARKER
+single_input: NEWLINE | simple_stmt | compound_stmt NEWLINE
 eval_input: testlist NEWLINE* ENDMARKER
 
 decorator: '@' dotted_name [ '(' [arglist] ')' ] NEWLINE