Add comments to type helper

helper/types.js

@@ -14,7 +14,9 @@ module.exports = function calculate_types(clean_types) {
     return undefined;
   }
 
-
+  /* the layers and source parameters are cumulative:
+   * perform a set insersection of their specified types
+   */
   if (clean_types.from_layers || clean_types.from_source) {
     var types = valid_types;
 
@@ -29,6 +31,10 @@ module.exports = function calculate_types(clean_types) {
     return types;
   }
 
+  /*
+   * Type restrictions requested by the address parser should only be used
+   * if both the source and layers parameters are empty, so do this last
+   */
   if (clean_types.from_address_parser) {
     return clean_types.from_address_parser;
   }