Conveyor Splitters/Mergers can be implemented as a box with inputs and outputs around it. To prevent the box from being filled with a full stack, the input(s) are wired with circuits to only be enabled when the box is empty.
Smart Splitter has a UI to set one filter on each output. Programmable Splitter allows up to 5 (Factorio's limit).
Any Undefined can just negate whatever filters are set, up to a limit of 5 (UI should show a warning that it won't work if this limit is exceeded between the other two outputs)
Overflow can be implemented by wiring the output inserter to the other ones, reading the other hand contents and activating if all other inserters are full of stuff.
Conveyor Splitters/Mergers can be implemented as a box with inputs and outputs around it. To prevent the box from being filled with a full stack, the input(s) are wired with circuits to only be enabled when the box is empty.
Smart Splitter has a UI to set one filter on each output. Programmable Splitter allows up to 5 (Factorio's limit).
Any Undefined can just negate whatever filters are set, up to a limit of 5 (UI should show a warning that it won't work if this limit is exceeded between the other two outputs)
Overflow can be implemented by wiring the output inserter to the other ones, reading the other hand contents and activating if all other inserters are full of stuff.