--- a/mercurial/revset.py	Sat Jun 28 01:42:39 2014 +0200
+++ b/mercurial/revset.py	Sat Jul 12 00:31:36 2014 -0700
@@ -1867,7 +1867,7 @@
         wb, tb = optimize(x[2], True)
 
         # (::x and not ::y)/(not ::y and ::x) have a fast path
-        def ismissingancestors(revs, bases):
+        def isonly(revs, bases):
             return (
                 revs[0] == 'func'
                 and getstring(revs[1], _('not a symbol')) == 'ancestors'
@@ -1876,12 +1876,10 @@
                 and getstring(bases[1][1], _('not a symbol')) == 'ancestors')
 
         w = min(wa, wb)
-        if ismissingancestors(ta, tb):
-            return w, ('func', ('symbol', '_missingancestors'),
-                       ('list', ta[2], tb[1][2]))
-        if ismissingancestors(tb, ta):
-            return w, ('func', ('symbol', '_missingancestors'),
-                       ('list', tb[2], ta[1][2]))
+        if isonly(ta, tb):
+            return w, ('func', ('symbol', 'only'), ('list', ta[2], tb[1][2]))
+        if isonly(tb, ta):
+            return w, ('func', ('symbol', 'only'), ('list', tb[2], ta[1][2]))
 
         if wa > wb:
             return w, (op, tb, ta)

--- a/tests/test-revset.t	Sat Jun 28 01:42:39 2014 +0200
+++ b/tests/test-revset.t	Sat Jul 12 00:31:36 2014 -0700
@@ -562,7 +562,7 @@
   5
   8
 
-check that conversion to _missingancestors works
+check that conversion to only works
   $ try --optimize '::3 - ::1'
   (minus
     (dagrangepre
@@ -571,7 +571,7 @@
       ('symbol', '1')))
   * optimized:
   (func
-    ('symbol', '_missingancestors')
+    ('symbol', 'only')
     (list
       ('symbol', '3')
       ('symbol', '1')))
@@ -586,7 +586,7 @@
       ('symbol', '3')))
   * optimized:
   (func
-    ('symbol', '_missingancestors')
+    ('symbol', 'only')
     (list
       ('symbol', '1')
       ('symbol', '3')))
@@ -599,7 +599,7 @@
       ('symbol', '6')))
   * optimized:
   (func
-    ('symbol', '_missingancestors')
+    ('symbol', 'only')
     (list
       ('symbol', '6')
       ('symbol', '2')))
@@ -618,7 +618,7 @@
         ('symbol', '4'))))
   * optimized:
   (func
-    ('symbol', '_missingancestors')
+    ('symbol', 'only')
     (list
       ('symbol', '6')
       ('symbol', '4')))