bugfix: revert removal of parentheses

introduced in: 87143e0dcabf312db8109f3a27764697f10ac2cd

It is caused as follows: `zip` produces a `list` of tuples and `map` feeds
each 2-`tuple` to the `lambda` function stored in `lambda_linspace`. So
writing `lambda (dom, res):` was correctly unpacking the 2-`tuple`, whereas
`lambda dom, res:` fails, because `lambda` will receive each time a single
argument, the 2-`tuple` given to it by `map`.