BeginPackage["Collatz`"]

Collatz::usage = "Collatz[n] gives a list of the iterates in the 3n+1
problem,
         starting from n. The conjecture is that this sequence always
         terminates."

Begin["`Private`"]

Collatz[1] := {1}

Collatz[n_Integer] := Prepend[Collatz[3 n + 1], n] /; OddQ[n] && n > 0

Collatz[n_Integer] := Prepend[Collatz[n/2], n] /; EvenQ[n] && n > 0

End[]

EndPackage[]

Get["DesktopCollatz.m"]

(* ::Package:: *)

(* ::Input:: *)
(*BeginPackage["Collatz`"]*)
(**)
(*Collatz::usage="Collatz[n] gives a list of the iterates in the 3n+1 problem,*)
(*        starting from n. The conjecture is that this sequence always*)
(*        terminates."*)
(**)
(*Begin["`Private`"]*)
(**)
(*Collatz[1]:={1}*)
(**)
(*Collatz[n_Integer]:=Prepend[Collatz[3 n+1],n]/;OddQ[n]&&n>0*)
(**)
(*Collatz[n_Integer]:=Prepend[Collatz[n/2],n]/;EvenQ[n]&&n>0*)
(**)
(*End[]*)
(**)
(*EndPackage[]*)