
theorem Th2:
  for a,b,c,d being set holds Indices (a,b)][(c,d) = {[1,1],[1,2], [2,1],[2,2]}
proof
  let a,b,c,d be set;
  thus Indices (a,b)][(c,d) = [:Seg 2,Seg 2:] by MATRIX_0:47
    .= [:{1},Seg 2:] \/ [:{2},Seg 2:] by FINSEQ_1:2,ZFMISC_1:109
    .= [:{1},Seg 2:] \/ {[2,1], [2,2]} by FINSEQ_1:2,ZFMISC_1:30
    .= { [1,1], [1,2]} \/ {[2,1], [2,2]} by FINSEQ_1:2,ZFMISC_1:30
    .= {[1,1],[1,2],[2,1],[2,2]} by ENUMSET1:5;
end;
