define Claim

similar to #21 based on https://github.com/valueflows/exchange/issues/14#issuecomment-216770713

as part of exchange where you have already consumed service of the other person (eg. got your car fixed)

In that case, you have a claim, which is stronger than a commitment. If you do your part of a set of reciprocal commitments, then you have a claim for the reciprocal event to happen.