Coding style.

- Name the sections of fill3.

fill3/fill3/__init__.py

@@ -16,6 +16,9 @@ import termstr
 __version__ = "v2022.01.05"
 
 
+##########################
+# Widgets
+
 def appearance_is_valid(appearance):
     return (all(isinstance(line, (str, termstr.TermStr)) and len(line) > 0 for line in appearance)
             and len(set(len(line) for line in appearance)) < 2)
@@ -407,7 +410,7 @@ class Fixed:
 
 
 ##########################
-
+# Infrastructure
 
 _EXCEPTION = None
 _LAST_APPEARANCE = []
@@ -492,8 +495,9 @@ async def tui(title, screen_widget):
     if _EXCEPTION is not None:
         raise _EXCEPTION
 
-##########################
 
+##########################
+# Example application
 
 class _Screen: