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