code: def _parse_arg(self, var, arg, scope):
          
          if isinstance(var, Variable):
              # kwarg
              if arg:
                  if utility.is_variable(arg[0]):
                      tmp = scope.variables(arg[0])
                      if not tmp:
                          return None
                      val = tmp.value
                  else:
                      val = arg
                  var = Variable(var.tokens[:-1] + [val])
          else:
              # arg
              if utility.is_variable(var):
                  if arg is None:
                      raise SyntaxError('Missing argument to mixin')
                  elif utility.is_variable(arg[0]):
                      tmp = scope.variables(arg[0])
                      if not tmp:
                          return None
                      val = tmp.value
                  else:
                      val = arg
                  var = Variable([var, None, val])
              else:
                  return None
          return var
docs: Parse a single argument to mixin.
          args:
              var (Variable object): variable
              arg (mixed): argument
              scope (Scope object): current scope
          returns:
              Variable object or None