Hi Matthias,
As per my understanding, the opts.Upper parameter only allows you to set upper bounds on the parameters values. It should allow you to set the upper limit of d as b. I would advise you to double check your syntax and that you have provided the correct order in the opts.Upper option.
You can also refer to this documentation to verify whether you are following the correct steps.
However one other way to work around this is you can set the ‘d’ value to be much larger than the expected value and in the fitting function, you can manually enforce the constraints that ‘d’ must be less than ‘b’.
