theorem
  {C1,C2} before {C} = {C1^C, C2^C}
proof
  thus
  {C1,C2} before {C} = ({C1} \/ {C2}) before {C} by ENUMSET1:1
    .= ({C1} before {C}) \/ ({C2} before {C}) by Th29
    .= {C1^C} \/ ({C2} before {C}) by Th31
    .= {C1^C} \/ {C2^C} by Th31
    .= {C1^C, C2^C} by ENUMSET1:1;
end;
