A final facility that modifies the set of reductions performed by presolve is the CPXcopyprotected routine. The user provides as input a list of variables in the original problem that should survive presolve (that is, should exist in the presolved problem). Presolve will avoid reductions that remove those variables, with one exception. If a protected variable can be fixed, presolve will still remove it from the problem. This command is useful in cases where the user wants to explicitly control some aspect of the branch & cut process (for example, through the branch callback) using knowledge about those variables.