docs: function Description(def) {
    if(typeof def === 'string') {
      this.parse(def);
    }else if(def
      && typeof def === 'object'
      && typeof def.txt === 'string'
      && typeof def.md === 'string') {
      this.md = def.md;
      this.txt = def.txt;
    }else{
      throw new Error('invalid value for description');
    }
  }