theorem
  M,v |= p => p '&' p & M |= p => p '&' p
proof
  now
    let v;
    M,v |= p implies M,v |= p '&' p by ZF_MODEL:15;
    hence M,v |= p => p '&' p by ZF_MODEL:18;
  end;
  hence thesis;
end;
