Document the decision

tests/test_grammar.py

@@ -3,7 +3,18 @@ import pytest
 
 
 def test_conflicting_names():
-    """Terminals and nonterminals can have the same name."""
+    """Terminals and nonterminals cannot have the same name.
+
+    I think that ultimately this gives a nicer experience, in error messages and
+    understandability. The input grammar can distinguish between them throughout,
+    and the system can always be unambiguous when it's working, but at times it
+    needs to report errors or display the grammar to humans. There is no clean
+    notation I can use at that time to distinguish between a terminal an a
+    nonterminal.
+
+    I think this restriction ultimately makes the grammars and the tooling easier
+    to understand.
+    """
 
     IDENTIFIER = parser.Terminal("Identifier")