sezrurkle lujvo

x1 is an open set in the topological space x2.

x2 is conceptually composed of an underlying set, which is a superset of x1, and topological information/defining information; however, it can also be expressed as a set of some of the subsets of the said underlying (implicitly define) set. Confer: "sezbarkle".

In notes:

x1 (metric space/set/class/structure) is a metric subspace/subset(/subclass) of x2 (metric space/set/class) which is closed therein. x1 is an closed subset of x2, where closedness is taken to be understood as being considered within x2 and under the metric shared by x1 and x2