code: def S_crosssection(x, group):
      """inner covariance matrix for White on group sums sandwich
  
      I guess for a single categorical group only,
      categorical group, can also be the product/intersection of groups
  
      This is used by cov_cluster and indirectly verified
  
      """
      x_group_sums = group_sums(x, group).T  # TODO: why transposed
  
      return S_white_simple(x_group_sums)
  
imports: import numpy
  from statsmodels.tools.grouputils import combine_indices, group_sums
  from statsmodels.stats.moment_helpers import se_cov
label_desc: Use image node_id 14 for calling a global function with example usage: S_crosssection(x, group) and returns: S_white_simple