[Python] Is there a way to cumulatively and distinctively expand list in polars

For distance, I want to accomplish conversion like below.

df = pl.DataFrame({
"col": [["a"], ["a", "b"], ["c"]]
})


┌────────────┐
│ col │
│ --- │
│ list[str] │
╞════════════╡
│ ["a"] │
│ ["a", "b"] │
│ ["c"] │
└────────────┘
↓
↓
↓
┌────────────┬─────────────────┐
│ col ┆ col_cum │
│ --- ┆ --- │
│ list[str] ┆ list[str] │
╞════════════╪═════════════════╡
│ ["a"] ┆ ["a"] │
│ ["a", "b"] ┆ ["a", "b"] │
│ ["c"] ┆ ["a", "b", "c"] │
└────────────┴─────────────────┘


I've tried polars.Expr.cumulative_eval(), but could not get it to work.

I can access the first element and last element in every iteration. But I want here is the result of the previous iteration i think. Could I get some help?

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