Keepsorted function

I'm trying to solve a problem on a function called "keepsorted";

This function has to keep a sub-list of growing number form a list called l1.

For example if I have let l1 = [1; 2; 0; 1; 2; 4; 3; 5; 3; 2; 6] the return is a list with the numbers [1; 2; 2; 4; 5; 6]

I tried many solution but I don't understand.

My last solution is:

let rec keepsorted l =   match l with  | [] -> []   | a1::a2::r when a1 >= a2 -> a1::keepsorted r   | a::r -> a::keepsorted r;;

and the result is:

- : int list = [1; 2; 1; 2; 4; 5; 2; 6]