let A, B be complex-membered set ; :: thesis: for a being Complex st a ++ A = a ++ B holds
A = B
let a be Complex; :: thesis: ( a ++ A = a ++ B implies A = B )
assume a ++ A = a ++ B ; :: thesis: A = B
then ( A c= B & B c= A ) by Th144;
hence A = B ; :: thesis: verum