let R be SimpleGraph; :: thesis: for A being StableSet of R
for B being Subset of A holds B is StableSet of R
let A be StableSet of R; :: thesis: for B being Subset of A holds B is StableSet of R
let B be Subset of A; :: thesis: B is StableSet of R
set VR = Vertices R;
reconsider BB = B as Subset of (Vertices R) by XBOOLE_1:1;
BB is stable by Def19;
hence B is StableSet of R ; :: thesis: verum