theorem
  M,v |= q => p 'or' q & M |= q => p 'or' q
proof
  now
    let v;
    M,v |= q implies M,v |= p 'or' q by ZF_MODEL:17;
    hence M,v |= q => p 'or' q by ZF_MODEL:18;
  end;
  hence thesis;
end;
