SGML vs XML

Hi,
     I have many times read when i am going through the XML tutorials that
SGML is to complex to be to be implem,ented for the web senario but i have
not found any page saying that ok XML is subset of SGML and these these are
the features which are there in SGML but are been excluded from the XML for
removing the complexity of the SGML.Can you please send me those features
which are not beening included in XML which are there for SGML??
Waiting for the reply
Sweta



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