P70 - Tree construction from a node string.

We suppose that the nodes of a multiway tree contain single characters.  In the depth-first order sequence of its nodes, a special character ^ has been inserted whenever, during the tree traversal, the move is a backtrack to the previous level.

By this rule, the tree in the figure opposite is represented as:

Define the syntax of the string and write a function string2MTree to construct an MTree from a String.  Make the function an implicit conversion from String.  Write the reverse function, and make it the toString method of MTree.