A derived variable can now be built on one with a lower VARIABLE_ORDER, so
the pages describe the order column, the ordered rule (replacing the
self-contained one) and a chained example.
New Configuration page explaining derived (computed) variables: what they
are, how to configure them in MPE_DERIVED, how filtering works, the rules
and limits (expressions only, self-contained, read-only, not stored), and
the data-exposure and performance notes.
New MPE_DERIVED table page, registered in the Table Guide nav.
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