If a person forgets a part of the other surah or is unable to complete it due to some compelling reason, such as shortness of time, he can leave that surah and recite another surah instead even if he has reached half of it and even if the surah he is reciting is al-Ikhlāṣ or al-Kāfirūn. In the case of forgetfulness, he can suffice with the amount he has recited [and he does not need to recite another full surah].