Regex failure on feature `for`

[Aristide]

The following program:

for (x, y) in seq:
    pass

is labelled as:

| `for:x` | 1 |
| `for:y` | 2 |

Instead of:

| `for:x` | 1-2 |
| `for:y` | 1-2 |

Strangely, it works for for x in seq and for (a, (b, c)) in seq.

This wrong labelling has an impact on all derived features, currently:

• accumulate_elements
• accumulate_some_elements
• find_best_element
• find_first_element
• for_range
• loop
• nested_for
• universal_quantification
• existential_quantification