[qbnztxps] Modular tetration

Can (tetrate(x,y) mod n) be calculated efficiently?  tetrate(x,y) is the power tower of x^x^x...^x of height y.

Sometimes very easily yes: tetrate(10,n) mod 3 = 10^x mod 3 = 1^x mod 3 = 1.  Similarly mod 5 would result in 0.