fiat_25519_addcarryx_u26
fiat_25519_addcarryx_u26(&x22, &x23, 0x0, x1, (x21 & UINT32_C(0x3ffffed)));
fiat_25519_addcarryx_u26(&x26, &x27, x25, x5, (x21 & UINT32_C(0x3ffffff)));
fiat_25519_addcarryx_u26(&x30, &x31, x29, x9, (x21 & UINT32_C(0x3ffffff)));
fiat_25519_addcarryx_u26(&x34, &x35, x33, x13, (x21 & UINT32_C(0x3ffffff)));
fiat_25519_addcarryx_u26(&x38, &x39, x37, x17, (x21 & UINT32_C(0x3ffffff)));