theorem th261bq:
  M |= F iff M |=0 ('G' F)
proof
  hereby assume Z1: M |= F;
    thus M |=0 ('G' F)
    proof
      let A;
      assume A in 'G' F;then
      consider B such that
A1:   A = 'G' B & B in F;
      thus M |=0 A by A1,th261b,Z1;
    end;
  end;
  assume Z1: M |=0 ('G' F);
  let A;
  assume A in F;then
  'G' A in 'G' F;
  hence M |= A by th261b,Z1;
end;
