setRotation method

void setRotation (double radians)

Turns the matrix into a rotation of radians

Implementation 

void setRotation(double radians) {
  final double c = math.cos(radians);
  final double s = math.sin(radians);
  _m2storage[0] = c;
  _m2storage[1] = s;
  _m2storage[2] = -s;
  _m2storage[3] = c;
}