Take cumsum of each row in polars

E.g. if I have

import polars as pl
df = pl.DataFrame({'a': [1,2,3], 'b': [4,5,6]})

how would I find the cumulative sum of each row?

Expected output:

    a   b
0   1   5
1   2   7
2   3   9

Here's the equivalent in pandas:

>>> import pandas as pd
>>> pd.DataFrame({'a': [1,2,3], 'b': [4,5,6]}).cumsum(axis=1)
    a   b
0   1   5
1   2   7
2   3   9

but I can't figure out how to do it in polars

cum_sum_horizontal() generates a struct of cum_sum values.

df.select(pl.cum_sum_horizontal(pl.all()))

shape: (3, 1)
┌───────────┐
│ cum_sum   │
│ ---       │
│ struct[2] │
╞═══════════╡
│ {1,5}     │
│ {2,7}     │
│ {3,9}     │
└───────────┘

Which you can unnest()

df.select(pl.cum_sum_horizontal(pl.all())).unnest('cum_sum')

shape: (3, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 5   │
│ 2   ┆ 7   │
│ 3   ┆ 9   │
└─────┴─────┘

The code for cum_sum_horizontal is here. As it stands, it just calls cum_fold()

df.select(pl.cum_fold(pl.lit(0, pl.UInt32), lambda x, y: x + y, pl.all()))

shape: (3, 1)
┌───────────┐
│ cum_fold  │
│ ---       │
│ struct[2] │
╞═══════════╡
│ {1,5}     │
│ {2,7}     │
│ {3,9}     │
└───────────┘