let R be RelStr ; :: 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
reconsider BB = B as Subset of R by XBOOLE_1:1;
BB is stable by Def2;
hence B is StableSet of R ; :: thesis: verum