Used when object of the bound is contained in the subject knot. On the first sight, contains predicate is confusingly similar to is_parent
However, while objects of ``is_parent'' relations can be themselves ``non-terminal'' - that is, they can themselves be subjects of other ``is_parent relations'', objects of bounds predicated with contains should always be terminals (i.e. should not be subjects of other contains relations).
Implemented to allow for possibility to have one and same recording included in several distinct container knots. Currently used mostly by the play template.