20241209
  Recant a change that appeared in version 20240825 because it caused
trouble in a large example.  Smaller examples of the need for this
change are so far unknown.
  Change internal handling of linear defined variables, sometimes
making handling of mpsi.mod
(https://www.ampl.com/ampl/models/mpsi.mod) much faster.  The old
internal handling is used if the "8" bit of option substout is on
(e.g., "option substout 8;" or "option substout 9;").  Processing
of mpsi1.mod (https://www.ampl.com/ampl/models/mpsi1.mod) is still
a bit faster, but not dramatically so.