function multiply(a, b, dst) {
  dst = dst || new MatType(16);
  var a00 = a[0];
  var a01 = a[1];
  var a02 = a[2];
  var a03 = a[3];
  var a10 = a[4 + 0];
  var a11 = a[4 + 1];
  var a12 = a[4 + 2];
  var a13 = a[4 + 3];
  var a20 = a[8 + 0];
  var a21 = a[8 + 1];
  var a22 = a[8 + 2];
  var a23 = a[8 + 3];
  var a30 = a[12 + 0];
  var a31 = a[12 + 1];
  var a32 = a[12 + 2];
  var a33 = a[12 + 3];
  var b00 = b[0];
  var b01 = b[1];
  var b02 = b[2];
  var b03 = b[3];
  var b10 = b[4 + 0];
  var b11 = b[4 + 1];
  var b12 = b[4 + 2];
  var b13 = b[4 + 3];
  var b20 = b[8 + 0];
  var b21 = b[8 + 1];
  var b22 = b[8 + 2];
  var b23 = b[8 + 3];
  var b30 = b[12 + 0];
  var b31 = b[12 + 1];
  var b32 = b[12 + 2];
  var b33 = b[12 + 3];
  dst[0] = a00 * b00 + a10 * b01 + a20 * b02 + a30 * b03;
  dst[1] = a01 * b00 + a11 * b01 + a21 * b02 + a31 * b03;
  dst[2] = a02 * b00 + a12 * b01 + a22 * b02 + a32 * b03;
  dst[3] = a03 * b00 + a13 * b01 + a23 * b02 + a33 * b03;
  dst[4] = a00 * b10 + a10 * b11 + a20 * b12 + a30 * b13;
  dst[5] = a01 * b10 + a11 * b11 + a21 * b12 + a31 * b13;
  dst[6] = a02 * b10 + a12 * b11 + a22 * b12 + a32 * b13;
  dst[7] = a03 * b10 + a13 * b11 + a23 * b12 + a33 * b13;
  dst[8] = a00 * b20 + a10 * b21 + a20 * b22 + a30 * b23;
  dst[9] = a01 * b20 + a11 * b21 + a21 * b22 + a31 * b23;
  dst[10] = a02 * b20 + a12 * b21 + a22 * b22 + a32 * b23;
  dst[11] = a03 * b20 + a13 * b21 + a23 * b22 + a33 * b23;
  dst[12] = a00 * b30 + a10 * b31 + a20 * b32 + a30 * b33;
  dst[13] = a01 * b30 + a11 * b31 + a21 * b32 + a31 * b33;
  dst[14] = a02 * b30 + a12 * b31 + a22 * b32 + a32 * b33;
  dst[15] = a03 * b30 + a13 * b31 + a23 * b32 + a33 * b33;
  return dst;
}
Multiplies two 4-by-4 matrices with a on the left and b on the right
@param {module:twgl/m4.Mat4} a The matrix on the left.
@param {module:twgl/m4.Mat4} b The matrix on the right.
@param {module:twgl/m4.Mat4} [dst] matrix to hold result. If not passed a new one is created.
@return {module:twgl/m4.Mat4} The matrix product of a and b.
@memberOf module:twgl/m4