A method and system for generating a Directed Acyclic Graph (DAG) from an initial multi-chain, subject to a constraint. The initial multi-chain is expressed as a string serving as a current input string to which the constraint is subsequently applied. A provided string P expresses the constraint. P is applied to the current input string to generate at least one output string, wherein each generated output string violates the constraint to a lesser extent than does the input string or does not violate the constraint. Each generated output string violating the constraint serves as a current input string to which the constraint is subsequently applied. P is recursively applied to each current input string that had been determined from applying P previously, until applying P does not generate any more output strings violating the constraint. A set of the generated output strings not violating the constraint represents the DAG.