x1=k1 (mass/si'o) is an equivalence class within/is a substructure of x2 (mathematical structure/object/category/set endowed with operations and properties) with defining characteristic/property/for similarity/under equivalence relation x3=p3, with representative member x4=p1=p2, in geometry/by standard/under paradigm x5=p4
x5 may have some overlap with x2. See also: panra, klesi, terpanryziltolju'i, panryzilbri