About type parameters: [T <: Seq[String]]
<: means T must be a subclass of Seq[String];
In the code, T must be assigned to an object lower down the line;
So, in a word, the chain is one-way and can't be reversed;
<: <: <: <: <: <: <: <: <: <:
You can think of this as a value chain.
Once onset, it can't be reversed. It will forever be on one direction.