[xml] recursive entities

[Scott Murman <smurman nas nasa gov>]

our group would like to use recursive entity definitions, i.e.

<!DOCTYPE Scenario SYSTEM "Scenario.dtd" [
    <!ENTITY Alt "1.0">
    <!ENTITY Height "2.0 * &Alt;">
]>

such as example does not work with my current version of libxml2 (Alt 
does not get substituted into Height).  is this a restriction of the 
XML standard or a limitation of the libxml2 implementation?  is there 
some other (preferred) way of accomplishing this? Thanks.

-SM-