fixed_variable_treatment:

Determines how fixed variables should be handled. $\;$
The main difference between those options is that the starting point in the "make_constraint" case still has the fixed variables at their given values, whereas in the case "make_parameter" the functions are always evaluated with the fixed values for those variables. Also, for "relax_bounds", the fixing bound constraints are relaxed (according to" bound_relax_factor"). For both "make_constraints" and "relax_bounds", bound multipliers are computed for the fixed variables. The default value for this string option is "make_parameter".
Possible values:

• make_parameter: Remove fixed variable from optimization variables
• make_constraint: Add equality constraints fixing variables
• relax_bounds: Relax fixing bound constraints