code: def _ncdf(x):
      return (tf.math.erf(x / _SQRT_2) + 1) / 2
  
imports: from typing import Tuple
  import numpy
  import tensorflow.compat.v2
  from tf_quant_finance import types
  from tf_quant_finance.black_scholes import vanilla_prices
  from tf_quant_finance.math import gradient
  from tf_quant_finance.math import integration
  from tf_quant_finance.math.root_search import newton
label_desc: Use image node_id 14 for calling a global function with example usage: _ncdf(x) and returns: unknown