I have a human-friendly sheet with sparse hierarchical data:

SEASON  | FRUIT      | LETTER 
-----------------------------
Winter  |            |
        | Lemons     |
        |            | Delta
Summer  |            |
        |            | Alpha
        |            | Beta
        | Pears      |
        |            | Gamma

(Note how Alpha and Beta don't have a FRUIT entry.)

I want to generate a new column, using ARRAYFORMULA(), to contain a full "path" to the LETTER:

SEASON  | FRUIT      | LETTER | PATH
------------------------------------
Winter  |            |        | Winter//
        | Lemons     |        | Winter/Lemons/
        |            | Delta  | Winter/Lemons/Delta
Summer  |            |        | Summer//
        |            | Alpha  | Summer//Alpha
        |            | Beta   | Summer//Beta
        | Pears      |        | Summer/Pears/
        |            | Gamma  | Summer/Pears/Gamma

Please help me to understand how to write such ARRAYFORMULA().

I'm trying approach, based on answers in