Hello all, this is a problem which has haunted me for some time. If this is simply hillarious, please tell me so. Or it may be some well known unsolvable problem... An assembly process takes inputs and produces outputs. I could say a Process is a function canProduce :: [Input]->[Output]->Bool which tells me if the outputs can be produced from the inputs There may be a similar function which tells me if the inputs are completely consumed to procude the output. The inputs do not determine the exact outputs. Think of a Process which combines a List of Ints into pairs, such that the input ints are consumed and each input Int occurs in only one position in the output. There are many ways to do this. Still for any set of input Ints and output pairs I could decide if the output can be produced from the input. Likewise the Input is not determined by the output. There may be lots of choices from what I could build my output (buy from different vendors). When I know more about the inputs and outputs my choices get more and more limited. I would like to to pass inputs and/or outputs to "something" and I would like to get a "something" which is more restricted, but still essentially a thing which tells me if the outputs can be produced from the inputs. I just cannot find a way to even THINK about this problem in a reasonable general way. -- Martin