Changing DOCTYPE during runtime

Hello List,
          My xml contains <!DOCTYPE project PUBLIC
"-//XYZ Corporation//Project 1.3//EN"
"h:/xyz_project/dtd/project.dtd"> and to change the
hardcoded "h:", I declared a system variable PROJ_HOME
and I changed the declaration 
<!DOCTYPE project PUBLIC "-//XYZ Corporation//Project
1.3//EN" "$PROJ_HOME:/xyz_project/dtd/project.dtd">. 
I am setting PROJ_HOME during runtime but still the
same thing is not reflected in xml document.
Am I doing something wrong?

regards,
Ranjan





                
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