15  * kernel tan function on [-pi/4, pi/4], pi/4 ~ 0.7854
19  * -1/tan (if k= -1) is returned.
22  *	1. Since tan(-x) = -tan(x), we need only to consider positive x.
23  *	2. if x < 2^-28 (hx<0x3e300000 0), return x with inexact if x!=0.
30  * 	        |tan(x)         2     4            26   |     -59.2
31  * 	        |----- - (1+T1*x +T2*x +.... +T13*x    )| <= 2
43  *      4. For x in [0.67434,pi/4],  let y = pi/4 - x, then
44  *		tan(x) = tan(pi/4-y) = (1-tan(y))/(1+tan(y))
45  *		       = 1 - 2*(tan(y) - (tan(y)^2)/(1+tan(y)))
54     pio4 = _F_64(7.85398163397448278999e-01), /* 0x3FE921FB, 0x54442D18 */
55     pio4lo = _F_64(3.06161699786838301793e-17), /* 0x3C81A626, 0x33145C07 */
57         _F_64(3.33333333333334091986e-01), /* 0x3FD55555, 0x55555563 */
58         _F_64(1.33333333333201242699e-01), /* 0x3FC11111, 0x1110FE7A */
59         _F_64(5.39682539762260521377e-02), /* 0x3FABA1BA, 0x1BB341FE */
60         _F_64(2.18694882948595424599e-02), /* 0x3F9664F4, 0x8406D637 */
61         _F_64(8.86323982359930005737e-03), /* 0x3F8226E3, 0xE96E8493 */
62         _F_64(3.59207910759131235356e-03), /* 0x3F6D6D22, 0xC9560328 */
63         _F_64(1.45620945432529025516e-03), /* 0x3F57DBC8, 0xFEE08315 */
64         _F_64(5.88041240820264096874e-04), /* 0x3F4344D8, 0xF2F26501 */
65         _F_64(2.46463134818469906812e-04), /* 0x3F3026F7, 0x1A8D1068 */
66         _F_64(7.81794442939557092300e-05), /* 0x3F147E88, 0xA03792A6 */
67         _F_64(7.14072491382608190305e-05), /* 0x3F12B80F, 0x32F0A7E9 */
68         _F_64(-1.85586374855275456654e-05), /* 0xBEF375CB, 0xDB605373 */
69         _F_64(2.59073051863633712884e-05), /* 0x3EFB2A70, 0x74BF7AD4 */
75     __float64 z, r, v, w, s;  in __kernel_tan()  local
79     if (ix < 0x3e300000) { /* x < 2**-28 */  in __kernel_tan()
92                     v = y - (z - x);  in __kernel_tan()
93                     t = a = -one / w;  in __kernel_tan()
96                     return t + a * (s + t * v);  in __kernel_tan()
103             x = -x;  in __kernel_tan()
104             y = -y;  in __kernel_tan()
106         z = pio4 - x;  in __kernel_tan()
107         w = pio4lo - y;  in __kernel_tan()
118     v = z *  in __kernel_tan()
121     r = y + z * (s * (r + v) + y);  in __kernel_tan()
125         v = (__float64)iy;  in __kernel_tan()
126         return (__float64)(1 - ((hx >> 30) & 2)) *  in __kernel_tan()
127                (v - _F_64(2.0) * (x - (w * w / (w + v) - r)));  in __kernel_tan()
132 			   simply return -1.0/(x+r) here */  in __kernel_tan()
133         /*  compute -1.0/(x+r) accurately */  in __kernel_tan()
137         v = r - (z - x); /* z+v = r+x */  in __kernel_tan()
138         t = a = _F_64(-1.0) / w; /* a = _F_64(-1.0)/w */  in __kernel_tan()
141         return t + a * (s + t * v);  in __kernel_tan()