code: def _expected_improvement_for_norm_diff(norm_diff):
      """The expected improvement."""
      return norm_diff * normal_distro.cdf(
          norm_diff
      ) + normal_distro.pdf(norm_diff)
  
imports: from __future__ import division
  from argparse import Namespace
  from copy import copy
  import numpy
  from scipy.stats import norm
  from ..utils.general_utils import solve_lower_triangular
  from ..gp.gp_core import get_post_covar_from_raw_covar
  from ..exd.domains import EuclideanDomain
  from ..exd.exd_utils import maximise_with_method
label_desc: Use image node_id 17 for calling a global function with example usage: _expected_improvement_for_norm_diff(norm_diff) and returns: unknown