code: def _ndtr_single(a: float) -> float:
      x = a / 2**0.5
  
      if x < -1 / 2**0.5:
          y = 0.5 * math.erfc(-x)
      elif x < 1 / 2**0.5:
          y = 0.5 + 0.5 * math.erf(x)
      else:
          y = 1.0 - 0.5 * math.erfc(x)
  
      return y
  
imports: import functools
  import math
  import sys
  from typing import Callable
  from typing import Optional
  from typing import Union
  import numpy
  from optuna.samplers._tpe._erf import erf
label_desc: Use image node_id 3 for calling a global function with example usage: _ndtr_single(a) and returns: y