code: function(theta) {
          _TEMP.identity();
          _TEMP.matrix[1][1] = _TEMP.matrix[2][2] = Math.cos(theta);
          _TEMP.matrix[2][1] = Math.sin(theta);
          _TEMP.matrix[1][2] = -_TEMP.matrix[2][1];
          return this.multiplySelf(_TEMP);
      }
docs: Applies rotation about X to this matrix.
  
  @param theta
  rotation angle in radians
  @return itself