theorem
  {C} before {C1,C2} = {C^C1, C^C2}
proof
  thus
  {C} before {C1,C2} = {C} before ({C1} \/ {C2}) by ENUMSET1:1
    .= ({C} before {C1}) \/ ({C} before {C2}) by Th30
    .= {C^C1} \/ ({C} before {C2}) by Th31
    .= {C^C1} \/ {C^C2} by Th31
    .= {C^C1, C^C2} by ENUMSET1:1;
end;
