add_holder

Adds holder to the list of holders managed within this.

NOTE: if this already has a Holder with the same ID as holder, then holder will not be added to the set (even if holder's type or value is not the same as the one already in this).

Parameters:

this	a Set object
holder	a Holder object

Returns:

true if holder has been added to this (and FALSE if it has not been added because there is another Holder with the same ID)