code: def build_columns(self, X, verbose=False):
          
          columns = super(FactorTerm, self).build_columns(X, verbose=verbose)
          if self.coding == 'dummy':
              columns = columns[:, 1:]
  
          return columns
docs: construct the model matrix columns for the term
  
          Parameters
          ----------
          X : array-like
              Input dataset with n rows
  
          verbose : bool
              whether to show warnings
  
          Returns
          -------
          scipy sparse array with n rows