code: func (bb *Builder) SetShareExpiration(t time.Time) *Builder {
  	if bb.Type() != "claim" || bb.ClaimType() != ShareClaim {
  		panic("called SetShareExpiration on non-share")
  	}
  	if t.IsZero() {
  		delete(bb.m, "expires")
  	} else {
  		bb.m["expires"] = RFC3339FromTime(t)
  	}
  	return bb
  }
docs: // SetShareExpiration sets the expiration time on share claim.
  // It panics if bb isn't a "share" claim type.
  // If t is zero, the expiration is removed.