series for (1 - 1 / (1-x^(-1)))(1 - b / (1-x^(-2)y^(-1)))(1 - c / (1-x^(-3)y^(-2)))(1 - d / (1-x^(-4)y^(-3)))(1 - e / (1-x^(-5)y^(-4)))(1 - f / (1-x^(-6)y^(-5))) at x=0