code: function(b) {
              	var lu = this.lu();
              	b = lu.P.x(b);
              	var y = lu.L.forwardSubstitute(b);
              	var x = lu.U.backSubstitute(y);
              	return lu.P.x(x);
              	//return this.inv().x(b);
                  }
docs: solve a system of linear equations (work in progress)