Avoid AddSequence Exception

Suppose you are training a sequence to sequence model for translating from English to Hungarian with inputs like

<s> I will not buy this record, it is scratched <s/> Nem fogok vásárolni ezt a lemezt, azt karcos </s>
<s> My hovercraft is full of eels <s/> A légpárnás hajóm tele van angolnákkal </s>
...

using the LUSequenceReader. There's a chance you'll encounter the cryptic exception

EXCEPTION occurred: AddSequence: Sequences must be a least one frame long.

The way to avoid it is to ensure that the <s/> token is in your vocabulary and its position corresponds to the variable separatorRow (0-based) in your config file.