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if op == "+" then
r = a + b
elseif op == "-" then
r = a - b
elseif op == "*" then
r = a*b
elseif op == "/" then
r = a/b
else
error("invalid operation")
end
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if (conditon) then
--action #1
elseif (condition) then
--action #2
else
--action #3
end
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if condition_1 then
statement_1
elseif condition_2 then
statement_2
else
statement_3
end