NEIGHBOR function in RPL

[Joe Horn]

The HP-71B Math ROM has a nifty function called NEIGHBOR(X,Y) which returns the nearest machine-representable number to X in the direction towards Y. For example, NEIGHBOR(5,7) returns 5.00000000001, and NEIGHBOR(5,1) returns 4.99999999999.

I'm looking for a User RPL implementation of that function. All my attempts fail when X is a power of 10 and Y<X. For example, NEIGHBOR(1,0) should return 0.999999999999 (twelve 9's), but I'm only getting eleven 9's. It's driving me crazier.

Any and all insights will be greatly appreciated.

BTW, I want to use the NEIGHBOR function in an program to be called "UNROUND", which takes any input J (which contains N digits after the decimal point) and returns the smallest and greatest numbers which round to J when N RND is executed. Example: 0.123 UNROUND should yield 0.1225 and 0.123499999999 because those are the smallest and greatest numbers (respectively) which yield 0.123 when 3 RND is executed. If you already have such a program, I'd love to see that too.