theorem Th11:
  {a,b,c} on A implies {a,c,b} on A & {b,a,c} on A & {b,c,a} on A
  & {c,a,b} on A & {c,b,a} on A
proof
  assume
A1: {a,b,c} on A;
  then
A2: c on A by INCSP_1:2;
  a on A & b on A by A1,INCSP_1:2;
  hence thesis by A2,INCSP_1:2;
end;
