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In Turing's 1936 paper "On Computable Numbers",

For a Turing Machine $M$, $Inst(q_i S_j S_k L q_l ) $ means that if $M$ scans symbol $S_j $ under $m-configuration$ $q_i$, then the symbol on the square under scanner (with symbol $S_j $ ) is to be replaced by symbol $S_k$, and the scanner/header moves one unit $Left$, and its new $m-configuration$ becomes $q_l$.

If:

${R_S}_j(x,y)$ means "In the complete configuration $x$ (of $M$) the symbol on the square $y$ is $S_j$.

$I(x,y)$ means that "In the complete configuration $x$ (of $M$) the square $y$ is scanned".

${K_q}_m(x)$ means that "In the complete configuration $x$ (of $M$) the m-configuration is $q_m$".

$F(x,y)$ means that "$y$ is the immediate successor of $x$".

Then $Inst(q_i S_j S_k L q_l ) $ is to be an abbreviation for:

$(x,y,x',y')$ { ($ {R_S}_j(x,y) \,\&\, I(x,y)\,\&\, {K_q}_i(x) \,\&\, F(x,x') \,\&\, F(y',y)) \to $ $( I(x',y') \,\&\, {R_S}_k(x',y) \,\&\, {K_q}_l(x') \,\&\, F(y',z) ∨ [( {R_S}_0(x,z) \to {R_S}_0(x',z)) \,\&\, ({R_S}_1(x,z) \to {R_S}_1(x',z)) \,\&\, ... \,\&\, ({R_S}_M(x,z) \to {R_S}_M(x',z))])$

$ S_0, S_1, ..., S_M $ being the symbols $M$ can print.

I am unable to convince myself of the exact correctness of the above formulae w.r.t. to the meaning of $Inst(q_i S_j S_k L q_l ) $. More specifically, why do we have the following expression included? What is he trying to "cover" by including it?

$ ... \,\&\, F(y',z) ∨ [({R_S}_0(x,z) \to {R_S}_0(x',z)) \,\&\, ({R_S}_1(x,z) \to {R_S}_1(x',z)) \,\&\, ... \,\&\, ({R_S}_M(x,z) \to {R_S}_M(x',z))] $

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