code: func (c *Corpus) ForeachClaim(permaNode blob.Ref, at time.Time, fn func(*camtypes.Claim) bool) {
  	pm, ok := c.permanodes[permaNode]
  	if !ok {
  		return
  	}
  	for _, cl := range pm.Claims {
  		if !at.IsZero() && cl.Date.After(at) {
  			continue
  		}
  		if !fn(cl) {
  			return
  		}
  	}
  }
docs: // ForeachClaim calls fn for each claim of permaNode.
  // 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.