code: function(field) {
      if (this.vars.hasOwnProperty(field)) {
        if (!this.usage.hasOwnProperty(field)) {
          this.usage[field] = 0;
        }
        this.usage[field]++;
      }
    }
docs: Tracks field names used within the source,
  and keeps a running tally of their use count.
  @param {String} name of field to report on.