The node contains the expression inside the parentheses through the inherited child node collection.

 FormulaSyntaxTree syntaxTree = FormulaSyntaxTree.Parse("SUM((North January):(West March))");
 FunctionNode functionNode = (FunctionNode) syntaxTree.getRoot();
 OperatorNode rangeNode = (OperatorNode) functionNode.getChildren().get(0);
 ParenthesisNode parenthesisNode = (ParenthesisNode) rangeNode.getChildren().get(0);
 SyntaxNode innerExpression = parenthesisNode.getChildren().get(0);