class_docstr: A covariance model for quadratic inference function regression.
  
  The mat method returns a basis matrix B such that the inverse
  of the working covariance lies in the linear span of the
  basis matrices.
  
  Subclasses should set the number of basis matrices `num_terms`,
  so that `mat(d, j)` for j=0, ..., num_terms-1 gives the basis
  of dimension d.`
docstr: Returns the term'th basis matrix, which is a dim x dim
  matrix.
docstr_tok: ["Returns","the","term'th","basis","matrix",",","which","is","a","dim","x","dim","matrix","."]
code: def mat(self, dim, term):
      """
      Returns the term'th basis matrix, which is a dim x dim
      matrix.
      """
      raise NotImplementedError
  
imports: import numpy
  from collections import defaultdict
  import statsmodels.base.model
  from statsmodels.genmod import families
  from statsmodels.genmod.generalized_linear_model import GLM
  from statsmodels.genmod.families import links
  from statsmodels.genmod.families import varfuncs
  import statsmodels.regression.linear_model
  import statsmodels.base.wrapper
  from statsmodels.tools.decorators import cache_readonly
label_desc: Use image node_id 1 for calling the QIFCovariance obj's underlying member method code with example usage: obj.mat(dim, term) without return types