Where is the error? Should <d> be <b> or should </b> be </d>? Or perhaps they should both be <f> ... </f> tags? How is the parser to know? To be sure the XML is not only well formed but also correct, you will have to validate the XML according to a DTD or Schema definition.
A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James