docs: def _igam(a, x):
      """Left tail of incomplete Gamma function:
  
              inf.      k
       a  -x   -       x
      x  e     >   ----------
               -     -
              k=0   | (a+k+1)
      """
      # Compute  x**a * exp(-x) / Gamma(a)
      ax = math.exp(a * math.log(x) - x - math.lgamma(a))
  
      # Power series
      r = a
      c = 1.0
      ans = 1.0
  
      while True:
          r += 1.0
          c *= x/r
          ans += c
          if c / ans <= MACHEP:
              return ans * ax / a