small comment

lib/ast.ml

@@ -80,6 +80,8 @@ and sign =
   | Times
   | Divide
 
+
+(* pp = pretty print *)
 let rec pp_query fmt ast = 
   match ast with
   | Select(cols, table_exp) ->  Format.fprintf fmt "%s" (String.cat "SELECT " (pp_columns cols) ^ pp_table_expression table_exp)