code: function alternation() {
      var ret = this.seq(
          concatenation,
          this.manyf(this.seqf(
              this.manyf(c_wsp),
              this.stringf('/'),
              this.manyf(c_wsp),
              concatenation)));
      if (ret[2].length === 0) {
          return ret[1];
      }
  
      var alts = ret[2].map(function(el){ return el[4]; });
      return new ast.Alternation([].concat(ret[1]).concat(alts));
  }
docs: alternation    =  concatenation *(*c-wsp "/" *c-wsp concatenation)