When can content be removed from a content model? The answer depends
what we mean by "remove". The first aspect is whether the content is
completely removed or if the minimum and/or maximum number of
occurences of the content is reduced though possibly still allowed.
The second aspect is whether the content that is removed from the
definition is still allowed to occur in documents. In general, a newer
language can be forwards and backwards compatible with an older
language if the component is removed and still accepted. A newer
language can be forwards compatible with an older language if the
component is optional and is removed and not accepted. A newer
language can be backwards compatible with an older language if the
component is optional and is removed and not accepted only if the
producers do not produce the optional component. A newer language
cannot be backwards or forwards compatible with an older language
if the component is required. More Information
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