diff --git a/lib/carto/renderer.js b/lib/carto/renderer.js
index f9e8423..2297c19 100644
--- a/lib/carto/renderer.js
+++ b/lib/carto/renderer.js
@@ -225,6 +225,11 @@ carto.Renderer.prototype.render = function render(m, callback) {
 };
 
 // This function currently modifies 'current'
+//
+// @param {Array} the current list of rules
+// @param {Object} definition a Definition object to add to the rules
+// @param {Object} byFilter an object/dictionary of existing filters
+// @param {Object} env the current environment
 function addRules(current, definition, byFilter, env) {
     var newFilters = definition.filters,
         newRules = definition.rules,
@@ -282,7 +287,8 @@ function addRules(current, definition, byFilter, env) {
 // called either once per render (in the case of mss) or per layer
 // (for mml)
 //
-// definitions: a list of definitions objects that contain .rules
+// @param {Object} definitions: a list of definitions objects that contain .rules
+// @param {Object} env the environment
 //
 // result: an array of arrays is returned, in which each array refers to a
 // specific attachment
@@ -294,6 +300,8 @@ function inheritDefinitions(definitions, env) {
     var result = [];
     var current, previous, attachment;
 
+    // Evaluate the filters specified by each definition with the given
+    // environment to correctly resolve variable references
     definitions.forEach(function(d) {
         d.filters.ev(env);
     });