theorem
  (A?) \/ (B?) = (A \/ B)?
proof
  thus (A \/ B)? = {<%>E} \/ (A \/ B) by Th76
    .= (A \/ {<%>E}) \/ (B \/ {<%>E}) by XBOOLE_1:5
    .= (A?) \/ (B \/ {<%>E}) by Th76
    .= (A?) \/ (B?) by Th76;
end;
