inconclusive truncation strategies in encode_plus?
# ❓ Questions & Help

## Details

Not sure if this is a bug or missing feature or I just misunderstood something: specifically, I'm wondering whether a common truncation strategy is missing from [`encode_plus`](https://huggingface.co/transformers/main_classes/tokenizer.html#transformers.PreTrainedTokenizer.__call__). More specifically, if you have a input sequence pair consisting of two texts `a` and `b` and invoke `encode_plus` as follows:

``` 
encode_plus(text=a, text_pair=b, max_length=100, truncation=whatever)
```

It seems that there is no truncation strategy that simply cuts of tokens from the end of the (internally concatenated) input sequence consisting of a and b. Instead the three options allow either to truncate from only a, from only b, or from the longest first (which could be either a or b, depending on the input). How can one remove token by token from the right until max_length is reached, e.g., also in the case that len(a)=200 and len(b)=2. In this example, no option seems to be suitable, e.g., "longest first" would remove only from a, "only first" likewise only from a (both of these remove from the first input a, but should in my case remove from end of the total sequence, e.g., first b then a if necessary), and "only second" only from b (which would be removed entirely, but since only second is defined, a will not be truncated so the total length is 200 and thus still longner than max length)

SO link: https://stackoverflow.com/questions/63280435/huggingface-transformers-truncation-strategy-in-encode-plus
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