code: func (c *Client) GetSigningIdentity(id string) (mspctx.SigningIdentity, error) {
  	im, _ := c.ctx.IdentityManager(c.orgName)
  	si, err := im.GetSigningIdentity(id)
  	if err != nil {
  		if err == mspctx.ErrUserNotFound {
  			return nil, ErrUserNotFound
  		}
  		return nil, err
  	}
  	return si, nil
  }
docs: // GetSigningIdentity returns signing identity for id
  //  Parameters:
  //  id is user id
  //
  //  Returns:
  //  signing identity