theorem th218:
  F |= A & (for B st B in F holds {}LTLB_WFF |= B)
    implies {}LTLB_WFF |= A
proof
  assume
Z1: F |= A & (for B st B in F holds {}LTLB_WFF |= B);
  let M;
  assume
Z2: M |= {}LTLB_WFF;
  now
    let B;
    assume B in F;then
    {}LTLB_WFF |= B by Z1;
    hence M |= B by Z2;
  end;then
  M |= F;
  hence M |= A by Z1;
end;
