Hi

I have formulated some constraints with parameters, something like the following:

```
% xg, yg are SX variables, p is SX parameter
constr_expr_h = [xg-p(1)-L1(1);...
-(xg- p(1))-L1(1);...
yg - p(2)-L1(2);...
-(yg - p(2))-L1(2)];
```

However i get the following error message stating that I have free variables. The goal is to assign these variables a value at each time step, which I have yet to figure out how.

```
Error using casadi.Function
Error in Function::Function for 'constr_fun' [SXFunction] at
.../casadi/core/function.cpp:240:
.../casadi/core/function_internal.cpp:146: Error calling SXFunction::init for
'constr_fun':
.../casadi/core/sx_function.cpp:476: constr_fun::init: Initialization failed since
variables [p_0p_1] are free. These symbols occur in the output expressions but you
forgot to declare these as inputs. Set option 'allow_free' to allow free
variables.
Error in detect_constr (line 86)
constr_fun = Function('constr_fun', {x, u, z}, {expr_constr});
Error in acados_ocp (line 121)
obj.model_struct = detect_constr(obj.model_struct, 0);
Error in RT_obs_acados_R1 (line 158)
ocp = acados_ocp(ocp_model,ocp_opts);
```

Thanks