code: public Matrix3 setToRotation (float angle) {
          float sina = FloatMath.sin(angle), cosa = FloatMath.cos(angle);
          return set(cosa, -sina, 0f,
                     sina, cosa, 0f,
                     0f, 0f, 1f);
      }
docs: Sets this to a rotation matrix.
  
  @return a reference to this matrix, for chaining.