Function reduce_exprs 

pub fn reduce_exprs<E>(
    f: PlanCallback<(Series, Series), Series>,
    exprs: E,
    returns_scalar: bool,
    return_dtype: Option<DataTypeExpr>,
) -> Expr
where
    E: AsRef<[Expr]>,

Analogous to Iterator::reduce.

An accumulator is initialized to the series given by the first expression in exprs, and then each subsequent value of the accumulator is computed from f(acc, next_expr_series). If exprs is empty, an error is returned when collect is called.