I don't know if similar already exists, but I needed to compress the belt's items in many situations.
The operation is simple
1) a constant combinator generates a ratio of different objects but with negative value
2) each belt with its own object only passes if its signal is negative
3) immediately after it is read how many objects have passed and the value is added to the initial signal
in short the system tries to bring to zero all the signals of constant combinator
4) a memory cell is necessary to store these operations
5) a decide combinator reset this process when all signals are zero or positive otherwise the system stops.
In the image you can see that the ratio for Green/Red/Blue circuit (1:4:3) are similar of output (70:279:209), more time passes, closer the ratio is
the system is extendable to how many signals can contain the constant combinator.
To Reset just set in the memory cell the mul "*" operation and immediately after restore the sum "+"