Validity of a single man and a woman praying in the first row behind the Imam

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If a single man and a woman stand behind the Imam, one view holds that their prayer is invalid because the woman cannot form a row with him since she cannot lead him. The sounder view, according to Ibn 'Aqil, is that it is valid because the man's prayer is valid standing next to someone whose leadership is established (even if only for the man, like a reader leading an illiterate person, or an immoral person leading the pious).