Simplex: outgoing variable cannot re-enter the basis next iteration

How can I prove that in the simplex method, a variable that has just left the basis cannot re-enter the basis on the very next iteration? The pivoting rule is Dantzig's.

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what is the pivot rule? – tulasi Jun 14 '12 at 16:00
with danzig rule thanks! – Yannis Assael Jun 15 '12 at 14:38

that the variable's $z_j - c_j$ becomes nonnegative in the next iteration? – BCLC Mar 7 at 20:38