--- a/checker.py	Sun Jul 03 23:15:56 2016 +0800
+++ b/checker.py	Sun Jul 03 23:27:49 2016 +0800
@@ -63,18 +63,33 @@
             return None
 
 
+def prevalidate(config):
+    ok = True
+
+    if 'url' not in config:
+        print("# C&O config doesn't have 'url' defined.")
+        ok = False
+
+    if 'scm' not in config:
+        print("# C&O config doesn't have 'scm' defined.")
+        ok = False
+    elif config['scm'] != 'hg':
+        print("# C&O checker doesn't support {} yet.".format(config['scm']))
+        ok = False
+
+    if not ok:
+        print('# C&O job failed')
+
+    return ok
+
+
 def execute(config):
     print('# C&O job started: {}'.format(now()))
     tmp = mkdtemp(prefix='candolint.')
     os.chdir(tmp)
 
-    ok = True
     source = './source'
-
-    if 'url' not in config:
-        print("# C&O config doesn't have url defined.")
-        print('# C&O job failed')
-        ok = False
+    ok = prevalidate(config)
 
     if ok:
         print('# C&O task: clone')