Presolve considerations
The default control settings of the FICO Xpress Optimizer presolver assume that the user is satisfied if the MIP search finds a single solution with the best objective. The choice of the default presolve settings attempts to improve the efficiency of this type of search by cutting off MIP solutions from the feasible region that are either degenerate (i.e., that have equivalent representations with different feasible values of the variables) or dominated (i.e., that can be deduced to be worse than solutions contained in the remaining feasible region) or symmetric to another solution. The user must take care to disable these default presolve operations when solutions may be removed from the enumeration search space that are of interest. Also, the user should be aware that disabling these presolve operations can significantly increase the enumeration runtime.
Presolve operations removing dominated MIP solutions are collectively referred to as dual reduction operations. To disable duplicate column detection the user must unset bit 5 of the integer control PRESOLVEOPS. To disable all dual reduction operations the user must unset bit 3 of PRESOLVEOPS. See the Applications: N-Best Solutions Example for an example of how to unset these bits.
To disable FICO Xpress Optimizer from removing symmetric solutions, set the SYMMETRY control to 0.
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