theorem Th38:
  FinJoin(B \/ {.b.},comp BL) = FinJoin (B,comp BL) "\/" b`
proof
  thus FinJoin(B \/ {.b.},comp BL) = FinJoin(B,comp BL) "\/" (comp BL).b by
Th13
    .= FinJoin (B,comp BL) "\/" b` by Def12;
end;
