How to create tree using web service?

I want to create tree structure using web service. I have used bottom up web service technique for creation of simple arithmatic operation. I can not understand, how to build a tree using web services. Is there is way to achieve this?

Use a recursive definition like :

expr = expr1 operand expr2 | node
expr1 = expr
expr2 = expr

operand = '+' | '-'

node = number

Then you cqn straightforward encode it using JSON or XML

15 + 3 - 2 becomes

{ "expr" : 
  { "expr1": { "expr1":"15" "operand":"+" "expr2":"3"} 
    "operand": "-"
    "expr2": "2"}}

or in xml

<expr>
   <expr1>
      <expr1>15</expr1>
      <operand>+</operand>
      <expr2>3</expr2>
   </expr1>
   <operand>+</operand>
   <expr2>2</expr2>
</expr>

In the service you can then walk the tree or build the tree. I am not sure if you see the service as a consumer or a producer.