The Gravity Stack

In a 'Gravity Stack', items are either Heavy (Uppercase) or Light (Lowercase). 1. A Heavy item added to the stack first 'crushes' (removes) all Light items currently at the top of the stack until it hits another Heavy item or the bottom. 2. A Light item can only be added to the stack if there is at least one Heavy item already in the stack to 'anchor' it. If the stack is empty or contains only Light items (which shouldn't happen), the incoming Light item is ignored. Input: A space-separated sequence of characters. Output: The final stack from bottom to top. Example: Input: b A c d B e Output: A B e (Explanation: 'b' is ignored (no Heavy). 'A' is pushed. 'c' and 'd' are pushed because 'A' is below. 'B' crushes 'c' and 'd', then 'B' is pushed. 'e' is pushed because 'B' is below.)