sezbarkle lujvo

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

In x2, x1 is composed exactly of members which are limit points of x1. Confer: sezrurkle.

In notes:

x1 is an open set in the topological space x2.