code: function (s) {
        var tree = []
        var value = ''
        var data
  
        // TODO Refactor, to get this removed.
        this.lastTreeInExpression = tree
        while (true) {
          data = this.lookUp(s.slice(value.length), value)
          if (data) {
            tree = tree.concat(data.tree)
            value = data.value
            this.lastTreeInExpression = tree
            if (!data.ret) {
              break
            }
          } else {
            break
          }
        }
        if (tree.length) {
          tree = this.composeExpression(tree)
        }
  
        return {tree: tree, value: value}
      }
docs: Parse expression.