Stores a value into the current collection with the specified key, overwriting the previous value associated with the key
public function set( Tk $k, Tv $v, ): this
If the key is not present, an exception is thrown. If you want to add
a value even if a key is not present, use
$coll->set($k,$v) is semantically equivalent to
$coll[$k] = $v
set() returns the current collection).
It returns the current collection, meaning changes made to the current collection will be reflected in the returned collection.
$k- The key to which we will set the value.
$v- The value to set.
this- Returns itself.