Internal parameters in R

Question

Sometimes I find R-functions that have undocumented parameters like use.names in c :

 x <- c(a = 1, b = 2, c = 3, d = 4) y <- c(e = 5, f = 6, g = 7, e = 8, h = 9) print(c(x, y)) # abcdefgeh # 1 2 3 4 5 6 7 8 9 print(c(x, y, i = 10)) # abcdefgehi # 1 2 3 4 5 6 7 8 9 10 print(c(x, y, use.names = F)) # [1] 1 2 3 4 5 6 7 8 9 

Is there a way to test this .Internal or .Primitive ? Are there any dangers in using them, for example, random changes in the API?