theorem
  FinMeet(B \/ {.b.},comp BL) = FinMeet (B,comp BL) "/\" b`
proof
  thus FinMeet(B \/ {.b.},comp BL) =FinMeet (B,comp BL) "/\" (comp BL).b by
Th20
    .=FinMeet (B,comp BL) "/\" b` by Def12;
end;
