Use of the apostrophe

The following program in a Pascal-like language may help

if plural(word)
then
    if possessive(word)
    then
        if lastchar(word) = "s"
        then word := word + "'"
        else word := word + "s'"
    else word := word + "s"
else
    if word = "it is"
    then word := "it's"
    else
        if possessive(word)
        then
            if word = "it"
            then word := "its"
            else
                if lastchar(word) = "s"
                then word := word + "'"
                else word := word + "'s"

Bob the Angry Flower's approach.