code: def S_within_hac(x, d, nlags=1, groupidx=1):
      """HAC for observations within a categorical group"""
      r = np.zeros(d.shape[1])
      r[0] = 1
      r[groupidx] = 1
      weights = weights_bartlett(nlags)
      return aggregate_cov(x, d, r=r, weights=weights)
  
file_path: statsmodels/statsmodels/sandbox/panel/sandwich_covariance_generic.py
imports: import numpy
label_desc: Use image node_id 5 for calling a global function with example usage: S_within_hac(x, d, nlags, groupidx) and returns: aggregate_cov