procedure SwapStrings(var S1, S2: string);
var
Tmp: string;
begin
Tmp := S1;
S1 := S2;
S2 := Tmp;
end;
var
S1, S2: string;
begin
S1 := 'A';
S2 := 'B';
SwapStrings(S1, S2);
//s1 is 'B', s2 is 'A'
WriteLn('s1 is ', S1, ', s2 is ', S2);
end.