local function insert(str, i, o)
    local first_half = str:sub(1, i - 1)
    local latter_half = str:sub(i, -1)
    return first_half .. o .. latter_half
end

local function tests()
    local res = insert("hello world", 1, "start: ")
    print(('insert("hello world", 1, "start: ") --> %s'):format(res))

    res = insert("hello world", 7, "small ")
    print(('insert("hello world", 7, "small ")  --> %s'):format(res))

    res = insert("hello world", 12, "!")
    print(('insert("hello world", 12, "!")      --> %s'):format(res))
end

tests()