Learn English – How to express you are not familiar with a field

In English, we use several metaphors to express this feeling which arises from being forbidden to speak.

• pent-up anger, trapped like an animal in a pen
• bottled up anger, trapped like a fluid in a sealed container
• stifled or smothered anger, stopped from healthy flow like a person who is suffocating

Bridle? Maybe. Literally, bridle, n., is the device for controlling a horse's head. Bridle, v., can mean to control a person or to control one's own visible reactions, as a rider controls a horse using a bridle (trans.). Or, it can mean to react with anger (not necessarily hidden) when one's will is interfered with (intrans.). The problem, then, is that bridled anger may be misunderstood to mean the act of bottling up an emotion, when what you meant was the resulting feeling.

Responding to the follow-up question about where we locate stifled anger: emotional reactions are located in the stomach not because of a cultural metaphor but because of a physical cause. Release of the stress hormone cortisol stimulates the body's “fight or flight” systems and disrupts other systems which might compete for resources, notably digestion. Extreme ongoing emotion can even cause evacuation of the digestive tract and bladder (the person may vomit, urinate, and defecate uncontrollably). This is thought to be an adaptation which enhances survival when one's life is threatened by an attacker.

In English, we often describe the experience of cortisol's effects as a visceral feeling, butterflies in the stomach, knot in the stomach, or sinking feeling in the pit of the stomach.

Several emotions can trigger the stress reaction, such as fear, anger, worry, and excitement. In the case of pent-up anger, the stress is continuous because the situation is unresolved, so we are likely to feel a knot in the stomach.

Saying that some number of slots are filled but that number could be any number between 0 and ALL means, simply, "Any number of slots could be filled. Or not."

The only real restriction you are placing on the description is that any slots that are filled are filled "front to back" or "first to last".

As far as "plain English", one phrase that will help is "so many". This simply means "an unknown number":

1. The first so many slots are filled. (PROBLEM: Heavily implies at least one.)

2. Slots should be filled first to last. (PROBLEM: Only notes the criteria.)

3. If slots are filled, they are the first so many slots. (Satisfies your criteria by specifically noting that none could be filled.)

4. Any slots filled are filled front to back. (Satisfies your criteria as this implies some number could be filled and it describes the condition.)

I think (4) is the easiest to understand and the most English-y but (3) is technically most similar to your given criteria.

You could also use "first to last" in (4) if that is more accurate.