docs: def get_num_primal_variables(self):
          """
          Gets number of primal variables.
  
          Returns
          -------
          num : int
          """
  
          if self.x is not None:
              return self.x.size
          if self.gphi is not None:
              return self.gphi.size
          if self.Hphi is not None:
              return self.Hphi.shape[0]
          if self.A is not None:
              return self.A.shape[1]
          if self.J is not None:
              return self.J.shape[1]
          if self.u is not None:
              return self.u.size
          if self.l is not None:
              return self.l.size
          return 0