@Rouben Rostamian  Amazing! That was exactly what I needed, thank you!

@Kitonum You definitely saved my day.

I would like to put thumbs up to your answers but I have not enough reputation yet. Please, wherever you are in the world, feel my gratitude coming towards you.

Thank you for all your effective support!

@Kitonum This seems to do almost exactly what I needed, but I didn't realized precisely what evalindets and degree functions do. I'll check documentation for that. 
However, your example does not work if there are coefficients multiplying the . How should I modify your command to take into account this case? Would any solution work also for parametric coefficients (e.g., theta)?

Thank you for all your support, you're extremely helpful!

@Kitonum Thank you for your answer. What about if w is an expression already given in advance, with terms in  appearing, and you want to write them as m[i,j]?