scanner: use infinitive verb after auxiliary word could

Could, as well as should, shall, must, may, can, might, etc.
are auxiliary words. After an auxiliary word should come an
infinitive verb.

tests/lib3/test_errors.py

@@ -0,0 +1,67 @@
+
+import yaml, test_emitter
+
+def test_loader_error(error_filename, verbose=False):
+    try:
+        list(yaml.load_all(open(error_filename, 'rb')))
+    except yaml.YAMLError as exc:
+        if verbose:
+            print("%s:" % exc.__class__.__name__, exc)
+    else:
+        raise AssertionError("expected an exception")
+
+test_loader_error.unittest = ['.loader-error']
+
+def test_loader_error_string(error_filename, verbose=False):
+    try:
+        list(yaml.load_all(open(error_filename, 'rb').read()))
+    except yaml.YAMLError as exc:
+        if verbose:
+            print("%s:" % exc.__class__.__name__, exc)
+    else:
+        raise AssertionError("expected an exception")
+
+test_loader_error_string.unittest = ['.loader-error']
+
+def test_loader_error_single(error_filename, verbose=False):
+    try:
+        yaml.load(open(error_filename, 'rb').read())
+    except yaml.YAMLError as exc:
+        if verbose:
+            print("%s:" % exc.__class__.__name__, exc)
+    else:
+        raise AssertionError("expected an exception")
+
+test_loader_error_single.unittest = ['.single-loader-error']
+
+def test_emitter_error(error_filename, verbose=False):
+    events = list(yaml.load(open(error_filename, 'rb'),
+                    Loader=test_emitter.EventsLoader))
+    try:
+        yaml.emit(events)
+    except yaml.YAMLError as exc:
+        if verbose:
+            print("%s:" % exc.__class__.__name__, exc)
+    else:
+        raise AssertionError("expected an exception")
+
+test_emitter_error.unittest = ['.emitter-error']
+
+def test_dumper_error(error_filename, verbose=False):
+    code = open(error_filename, 'rb').read()
+    try:
+        import yaml
+        from io import StringIO
+        exec(code)
+    except yaml.YAMLError as exc:
+        if verbose:
+            print("%s:" % exc.__class__.__name__, exc)
+    else:
+        raise AssertionError("expected an exception")
+
+test_dumper_error.unittest = ['.dumper-error']
+
+if __name__ == '__main__':
+    import test_appliance
+    test_appliance.run(globals())
+