Search Engine with Python

Hi there. I was doing some extra exercise apart from the treehouse track and I got this one. Given an ORDERED (ascendent order) list of tuples (pairs) where the first element is a word in a file and the second is the name of the file itself, the search engine should check for possible similar words between different files and at the end print a list with all the words coupled with the name of the file they belong. For instance, starting with a similar list ordered according to the word:

x = [['boat', 'a.txt'], ['boat', 'c.txt'], ['cat', 'b.txt'], ['dog', 'a.txt'], ['dog', 'b.txt'], ['dog', 'c.txt'], ['house', 'a.txt'], ['rat', 'c.txt']]

my output should be:

res = [['boat', ['a.txt', 'c.txt']], ['cat', ['b.txt']], ['dog', ['a.txt', 'b.txt', 'c.txt']], ['house', ['a.txt']], ['rat', ['c.txt']] 

I tried different ways without success. I am posting one of the versions I did. In this one, I am trying to separate the words and file name into two separate lists to append simultaneously to the res list. Unfortunately, I get an index error. I tried some fixing but I couldn't make it work.... I need some help, please

def make_table(pairs):
    res = []
    fresh = fpairs[0]
    i = 1
    k = 0
    while i < len(fpairs):

        if fpairs[i][0] != fresh:
            res[k][0].append(fresh[0])
            res[k][1].append(fresh[1])
            fresh = fpairs[i]
            k += 1
        else:
            res[k][1].append(fresh[1])
        i += 1

    res.append([word, file])

x = [['boat', 'a.txt'], ['boat', 'c.txt'], ['cat', 'b.txt'], ['dog', 'a.txt'], ['dog', 'b.txt'], ['dog', 'c.txt'], ['house', 'a.txt'], ['rat', 'c.txt']]
make_table(x)