code: def _ncdf(x):
      """Implements the cumulative normal distribution function."""
      return (tf.math.erf(x / _SQRT_2) + 1) / 2
  
file_path: tf-quant-finance/tf_quant_finance/models/hull_white/zero_coupon_bond_option.py
imports: from typing import Callable, Union
  import numpy
  import tensorflow.compat.v2
  from tf_quant_finance import types
  from tf_quant_finance import utils
  from tf_quant_finance.math import random
  from tf_quant_finance.models import utils
  from tf_quant_finance.models.hjm import zero_coupon_bond_option_util
  from tf_quant_finance.models.hull_white import one_factor
label_desc: Use image node_id 1 for calling a global function with example usage: _ncdf(x) and returns: unknown