Letter-based representation of pairwise comparisons

Hi!

I have been working several years with R but it's my first public question.
I hope I'll be clear :) .

This question is related to obtaining letter-based representation of
non-parametric pairwise comparisons.

I have a dataframe with this structure (but with quite more rows and cols):

A B C  factor
1 2 2 one
2 1 2 one
2 2 3 two
2 3 2 two
1 4 2 three
9 8 1 three

I have no normality, so I did a kruskal test which showed significant
differences in some cases. As post hoc pairwise comparisons (the idea is to
make an equivalent to Tukey test after an ANOVA) I tried with a paired
wilcoxon test with the Bonferroni's correction (pairwise.wilcox.test(A,
factor, p.adj="bonf") and other for B, and so on) and, now, I need to modify
the output to show a letter-based representation of all pairwise
comparisons. Some algorithms have been published
(http://www.accessmylibrary.com/coms2/summary_0286-21655824_ITM), but I
can't find an R implementation.