Do not clobber explicit annotations.

marrow/schema/meta.py

@@ -86,8 +86,11 @@ def process(name, attr):
 		# Iteratively process the Element subclass instances and update their definition.
 		attributes.update(process(k, v) for k, v in attrs.items() if isinstance(v, Element))
 		attrs['__attributes__'] = odict(sorted(attributes.items(), key=lambda t: t[1].__sequence__))
-		attrs.setdefault('__annotations__', dict())
-		attrs['__annotations__'].update({k: v.annotation for k, v in attributes.items() if hasattr(v, 'annotation')})
+		ann = attrs.setdefault('__annotations__', dict())
+
+		for k, v in attributes.items():
+			if not hasattr(v, 'annotation'): continue  # Skip missing or None annotations.
+			ann.setdefault(k, v.annotation)  # If an annotation is already set (explicitly by the developer), skip.
 
 		# Construct the new class.
 		cls = type.__new__(meta, str(name), bases, attrs)