text: #include <math.h>
  #if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
  long double logbl(long double x) { return logb(x); }
  #else
  long double logbl(long double x)
  {
      if (!isfinite(x))
          return x * x;
      if (x == 0)
          return -1 / (x * x);
      return ilogbl(x);
  }
  #endif
  