theorem Th20:
  FinMeet(B \/ {.b.},f) = FinMeet (B,f) "/\" f.b
proof
  set M= the L_meet of 1L;
  thus FinMeet (B \/ {.b.},f) = M $$ (B \/ {.b.},f) by LATTICE2:def 4
    .= M $$ (B,f) "/\" f.b by SETWISEO:32
    .= FinMeet (B,f) "/\" f.b by LATTICE2:def 4;
end;
