diff --git a/miplearn/instance.py b/miplearn/instance.py
index a90744f..a401e3a 100644
--- a/miplearn/instance.py
+++ b/miplearn/instance.py
@@ -59,11 +59,18 @@ class Instance(ABC):
         provide as variable features the weight and the price of a specific item.
         
         Like instance features, the arrays returned by this method MUST have the same length for
-        all variables, and for all relevant instances of the problem.
-        
-        If the value of the given variable should not be predicted, this method MUST return None.
+        all variables within the same category, for all relevant instances of the problem.
         """
         pass
 
     def get_variable_category(self, var, index):
+        """
+        Returns a category (a string, an integer or any hashable type) for each decision variable.
+        
+        If two variables have the same category, LearningSolver will use the same internal ML model
+        to predict the values of both variables. By default, all variables belong to the "default"
+        category, and therefore only one ML model is used for all variables.
+        
+        If the returned category is None, ML models will ignore the variable.
+        """
         return "default"
diff --git a/mkdocs.yml b/mkdocs.yml
index 5d0c202..e0dc1f2 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -1,6 +1,6 @@
 site_name: MIPLearn
 theme: cinder
-copyright: Copyright (C) 2019-2020 Argonne National Laboratory
+copyright: "© Copyright 2019–2020 Argonne National Laboratory"
 repo_url: https://github.com/iSoron/miplearn
 nav:
     - Home: index.md