Suppose we want to design a simple couch. THe imagined material is polyurethane foam. We can imagine two parallelepipeds, one flattened acts as the seat, the other shortened as the backrest. The formula is 

This operation in the Mental Design Model we would call an  addition 
its symbol is 

Note that this operation created fissures between the components of the object 

Now we take a slightly different approach to our couch design. We can imagine a parallelopids, and a metaphysical knife that slides through it cutting a sector. The result is also a couch.  The formula is 

This operation in the Mental Design Model we would call an  subtraction  its symbol is 

A third way to imagine the couch in our imagination is by bending a flattened cube  The formula is 

This operation in the Mental Design Model we would call a transformation:  its symbol is 

Note that in this operation all the edges are smooth, especially the one in the back. Also some wrinkles may appear between the seat sector and the backrest.