code: def _phi_1993(s, t, gamma, h, x, r, b, sigma):
      """Computes the value of the Phi (7) function in reference [1]."""
  
      # The naming convention will align variables with the variables named in
      # reference [1], but with X = _boundary_1993().
      # [1] https://core.ac.uk/download/pdf/30824897.pdf
  
      kappa = (2 * b) / sigma**2 + (2 * gamma - 1)
      d1 = -(
          tf.math.log(s / h) + (b + (gamma - 0.5) * sigma**2) * t
      ) / (sigma * tf.math.sqrt(t))
      d2 = -(
          tf.math.log(x**2 / (s * h))
          + (b + (gamma - 0.5) * sigma**2) * t
      ) / (sigma * tf.math.sqrt(t))
      lambd = -r + gamma * b + 0.5 * gamma * (gamma - 1) * sigma**2
  
      return (
          tf.math.exp(lambd * t)
          * s**gamma
          * (_ncdf(d1) - (x / s) ** kappa * _ncdf(d2))
      )
  
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 12 for calling a global function with example usage: _phi_1993(s, t, gamma, h, x, r, b, sigma) and returns: unknown