function _rotationMatrix2x2 (theta, format) {
    const Big = isBigNumber(theta)

    const minusOne = Big ? new BigNumber(-1) : -1
    const cosTheta = cos(theta)
    const sinTheta = sin(theta)
    const data = [[cosTheta, multiplyScalar(minusOne, sinTheta)], [sinTheta, cosTheta]]

    return _convertToFormat(data, format)
  }
Returns 2x2 matrix of 2D rotation of angle theta

@param {number | BigNumber | Complex | Unit} theta  The rotation angle
@param {string} format                              The result Matrix storage format
@returns {Matrix}
@private