code: function(){
          var node = this.parseAdd();
          while(this.skipValue(lexer.TOKEN_TILDE, '~')) {
              var node2 = this.parseAdd();
              node = new nodes.Concat(node.lineno,
                                   node.colno,
                                   node,
                                   node2);
          }
          return node;
      }
docs: finds the '~' for string concatenation