diff --git a/mercurial/repo.py b/mercurial/repo.py
--- a/mercurial/repo.py
+++ b/mercurial/repo.py
@@ -9,16 +9,26 @@
 class RepoError(Exception):
     pass
 
+class NoCapability(RepoError):
+    pass
+
 class repository(object):
     def capable(self, name):
         '''tell whether repo supports named capability.
         return False if not supported.
         if boolean capability, return True.
         if string capability, return string.'''
+        if name in self.capabilities:
+            return True
         name_eq = name + '='
         for cap in self.capabilities:
-            if name == cap:
-                return True
             if cap.startswith(name_eq):
                 return cap[len(name_eq):]
         return False
+
+    def requirecap(self, name, purpose):
+        '''raise an exception if the given capability is not present'''
+        if not self.capable(name):
+            raise NoCapability(_('cannot %s; remote repository does not '
+                                 'support the %r capability') %
+                               (purpose, name))