SUCCESIVE USE OF BACKWARD SUBSTITUTION

For any two statements S1 and S2,
if you have proved that

{F1} S1 {F2}

and

{F2} S2 {F3}

then you candeduce

{F1} S1; S2 {F3}

Proof rule in Hoare's notation:

{F1}S1{F2}, {F2}S2{F3}
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{F1}S1;S2{F3}