code: func (c *Corpus) ForeachClaimBack(value blob.Ref, at time.Time, fn func(*camtypes.Claim) bool) {
  	for _, cl := range c.claimBack[value] {
  		if !at.IsZero() && cl.Date.After(at) {
  			continue
  		}
  		if !fn(cl) {
  			return
  		}
  	}
  }
docs: // ForeachClaimBack calls fn for each claim with a value referencing br.
  // If at is zero, all claims are yielded.
  // If at is non-zero, claims after that point are skipped.
  // If fn returns false, iteration ends.
  // Iteration is in an undefined order.