006-parse_nested_parens does not validate that the computed depth is correct

[jesyspa]

The algorithm checks only that the depth is non-negative. res := []; would be a valid implementation.

Possible strengthenings:

• Prove that the split of a string gives a list of strings that join up to the starting string.
• Prove that parse_paren_group returns the maximum depth.
• Prove that parse_nested_parens gives an array that matches with the behaviour of split and parse_paren_group.

Note that this may require strengthening the precondition to ensure that the string really does consist of a number of groups.