Implement !<: operator

[marcelocantos]

Checklist:

• Added related tests
• Made corresponding changes to the documentation

[coveralls]

Pull Request Test Coverage Report for Build 37f998d44b37128200476414db874a58d07c3f4d-PR-260

• 7 of 8 (87.5%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.06%) to 42.198%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
syntax/compile.go	7	8	87.5%

Files with Coverage Reduction	New Missed Lines	%
syntax/import_cache.go	1	80.49%

Totals
Change from base Build f9a8973e27a18db55c2a8ae33f08863e26c7b21a:	0.06%
Covered Lines:	3126
Relevant Lines:	7408

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 7ff9ab58177cb182f330a22cfb2177652c94390f-PR-260

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 7 of 8 (87.5%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.008%) to 41.938%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
syntax/compile.go	7	8	87.5%

Files with Coverage Reduction	New Missed Lines	%
syntax/import_cache.go	1	80.49%
rel/value_set_generic.go	3	66.22%

Totals
Change from base Build a2ebd6262381fc68731a373f3d772c82337aa8dd:	0.008%
Covered Lines:	3095
Relevant Lines:	7380

---

💛 - Coveralls