In a number of places we use the term "overlap" to describe a relationship between trees (or their roots) in the sense that two trees overlap if they have nodes in common.

The term is unfortunate (a) because overlap implies a more complex relationship than containment, and (b) there's a whole literature devoted to overlap in markup which is to do with handling of non-hierarchic relationships, and our use of the term is quite unrelated to this.

See current discussion on xsl-list.