code: def add_var(self, var):
          
          if var.name in [v.name for v in self.vars]:
              logger.error("Variable set named '%s' already exists." % var.name)
              return
  
          var.i1 = self.var_N
          var.iN = self.var_N + var.N - 1
          self.vars.append(var)
docs: Adds a variable to the model.