[R] Nagelkerke Pseudo R-squared

[varin sacha]

Dear R-Experts,

I have fitted an ordinal logistic regression with just 1 explanatory variable for the reproducible example here below.

Everything is working, now I try to calculate the Nagelkerke Pseudo R-squared. 
I have found a package BaylorEdPsych providing many Pseudo R-squared, but the example shown in the package is for GLM (binary logistic regression) not for ordinal logistic regression.
How can I calculate the Nagelkerke Pseudo R-squared for ordinal logistic regression ?

Many thanks as usual for your precious help.

Reproducible example :

install.packages("MASS") 
library(MASS) 
a=factor(c("tres grand", "grand", "petit","petit","tres grand","grand","petit","petit","tres grand","grand")) 
b=c("homme", "homme", "femme", "femme", "femme", "homme", "homme", "homme", "femme", "femme") 
m <- polr(a ~ b, Hess=TRUE) 
summary(m)