def _evaluate_priority_symbol(sym)
      if (priority = _symbol_to_priority(sym))
        _evaluate_priority(priority)
      else
        raise Errors::InvalidPriorityError, sym.inspect
      end
    end
def _evaluate_priority_symbol(sym)
      if (priority = _symbol_to_priority(sym))
        _evaluate_priority(priority)
      else
        raise Errors::InvalidPriorityError, sym.inspect
      end
    end
Evaluate a symbol as a priority.