AND gate
= AND gate
{c}
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An <AND gate> computes $a\land b=ab$ for bits $a,b$. It is irreversible when only its one-bit output is retained. The <Toffoli gate> embeds the operation reversibly as $(a,b,c)\mapsto(a,b,c\oplus ab)$; setting $c=0$ preserves the inputs while recording their <logical conjunction>.