code: function(sass) {
      var match;
      while ((match = this.fieldRegexAll.exec(sass)) !== null) {
        if (this.vars.hasOwnProperty(match[1])) {
          this._addFieldUsage(match[1]);
        }
      }
      return this;
    }
docs: Counts the usage of all field identifiers in the source text.
  Field counts are reported into the parser's usage table.
  @param {String} sass string to count field usage in.