Thank you very much Werner. I remebered there was a way to do it, I think you helped me with another problem in the past using this type of solve bloc "outputing" a function, but I couldn't figure out how to set it up properly ! I will keep this sheet very preciously, as it is a very interesting method to solve a lot of problems.
My apologies, by the way, I should have specified that I was working for d in [0,2[ (d is actually the sum of 2 duty cycles, each one between 0 and 1). It is such an "obvious" hypothesis for me, because I am so focused on the problem, that it became kind of implicit. So you are right, it wouldn't work out of this interval.
You are also right that, in this example, the min in the x function is not necesary. In the application, I do use x with d in the whole range of d, so I just mechanically put the min whith application in mind. Once again, hypothesis that are implicit considering the application, but are absolutely not obvious when asking for help.
Next time, I will try to be more rigourous, and add these "implicit hypothesis".
Once again, thank you very much !