Add some more comments to validate_symbol_tables()

Author: saxbophone

basest/core/utils.py

@@ -42,10 +42,13 @@ def validate_symbol_tables(symbol_table, padding_symbol, other_symbol_table):
     Raises ValueError if either of the symbol tables (or padding symbol) fail
     validation.
     """
+    # first check that they all do not contain None
     if None in (symbol_table + [padding_symbol] + other_symbol_table):
         raise ValueError(
             'None cannot be used in symbol tables nor for padding'
         )
+    # if that check passes, validate tables (and padding) for uniqueness
+    # the padding symbol is evaluated with the first symbol table
     elif (
         (not symbol_table_is_unique(symbol_table, padding_symbol)) or
         (not symbol_table_is_unique(other_symbol_table))