<MvUNDELETE> removes a mark set with
<MvDELETE> from the current record in the database.
Undeleted records will NOT be physically deleted by <MvPACK>.
When a database is packed, physical record numbers (as provided by the recno built-in variable) are reset as
required. For this reason, you should not code in a way that uses the physical record number as a unique identifier by
which a record can be referred to. You may instead wish to define a database field in which you store a unique
identifier for each record. For example db_alias.d.id.