code: func (bb *Builder) SetShareTarget(t blob.Ref) *Builder {
  	if bb.Type() != "claim" || bb.ClaimType() != ShareClaim {
  		panic("called SetShareTarget on non-share")
  	}
  	bb.m["target"] = t.String()
  	return bb
  }
docs: // SetShareTarget sets the target of share claim.
  // It panics if bb isn't a "share" claim type.