fix: two issues with the N3 serializer

[aucampia]

Summary of changes

This patch fixes two issues with the N3 serializer:

• The N3 serializer incorrectly considered a subject as already
  serialized if it has been serialized inside a quoted graph.
• The N3 serializer does not consider that the predicate of
  a triple can also be a graph.

Other changes included in this patch:

• Changed test.testutils.GraphHelper to support nested/quoted graphs.
• Moved the tests from test/test_n3_formula.py into
  test/test_serializers/test_serializer_n3.py.
• Include positive syntax tests from the N3 test suite that is smaller
  than 1024KB and that is not using new N3 syntax into round trip tests.
  This is mainly to check that there is no regressions after the changes
  made.

Fixes:

• N3 serializer "randomly" omits triple #1807
• Strange bug of notation3 serialization that occur with “probability” #1701

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[ghost]

The N3 serializer does not consider that the predicate of a triple can also be a graph.

It can? Eww. Excellent catch.

[aucampia]

The N3 serializer does not consider that the predicate of a triple can also be a graph.

It can? Eww. Excellent catch.

It is a bit odd yes, noticed it when I ran roundtrip for this test:

https://github.com/w3c/N3/blob/c44d123c5958ca04117e28ca3769e2c0820f72e6/tests/N3Tests/manifest-parser.ttl#L1097-L1102

:cwm_syntax_neg-formula-predicate.n3
        a              test:TestN3PositiveSyntax ;
        mf:action      <cwm_syntax/neg-formula-predicate.n3> ;
        mf:name        "neg-formula-predicate.n3" ;
        rdfs:comment   "A predicate cannot be a formula, I hope ; yes it can!";
        rdft:approval  rdft:Approved .

File here:

https://github.com/w3c/N3/blob/c44d123c5958ca04117e28ca3769e2c0820f72e6/tests/N3Tests/cwm_syntax/neg-formula-predicate.n3

:a {:b :c :d} :e .

And the only reason it failed round tripping is because the namespace was not being serialized for the quad in the middle, with this fix the reconstituted graph looks like this:

DEBUG    test.test_roundtrip:test_roundtrip.py:237 source = /home/iwana/sw/d/github.com/iafork/rdflib.cleanish/test/data/suites/w3c/n3/N3Tests/cwm_syntax/neg-formula-predicate.n3, serailized = 
@prefix ns1: <file:///home/iwana/sw/d/github.com/iafork/rdflib.cleanish/test/data/suites/w3c/n3/N3Tests/cwm_syntax/neg-formula-predicate.n3#> .

ns1:a{
    ns1:b ns1:c ns1:d .

} ns1:e .

Without it:

DEBUG    test.test_roundtrip:test_roundtrip.py:237 source = /home/iwana/sw/d/github.com/iafork/rdflib.cleanish/test/data/suites/w3c/n3/N3Tests/cwm_syntax/neg-formula-predicate.n3, serailized = 

<file:///home/iwana/sw/d/github.com/iafork/rdflib.cleanish/test/data/suites/w3c/n3/N3Tests/cwm_syntax/neg-formula-predicate.n3#a>{
    ns1:b ns1:c ns1:d .

} ns1:e .

Had to double check the grammar to believe it also:

https://github.com/w3c/N3/blob/master/grammar/n3.ebnf

[14] predicate                          ::= (expression | '<-' expression)
                                            /* allow inverting first predicate in a path */
[16] expression                         ::= path

[17] path	                              ::= pathItem ('!' path | '^' path)?

[18] pathItem                           ::= iri
                                          | blankNode
                                          | quickVar
                                          | collection
                                          | blankNodePropertyList
                                          | literal
                                          | formula

[22] formula                            ::= '{' formulaContent? '}'

[23] formulaContent                     ::= n3Statement ('.' formulaContent?)?
                                          | sparqlDirective formulaContent?

[ghost]

The comment suggests it's perhaps not intended to be that way but is an entailment of the EBNF expression.

rdfs:comment   "A predicate cannot be a formula, I hope ; yes it can!";

[aucampia]

@eikeon not sure if you are familiar with the N3 serializer but if you are please have a look, I'm fairly confident about this change but maybe I'm missing something here also.

[aucampia]

@blackwint3r this ideally needs one more review, please consider doing a review if you have time.

[coveralls]

Coverage increased (+0.007%) to 88.255% when pulling f38979e on aucampia:iwana-20220419T2343-n3_serialize_quoted_graph into 5e5dcea on RDFLib:master.

[aucampia]

resolving conflicts

[aucampia]

@RDFLib/core I will merge this by 2022-05-17 if there is no further feedback.

[aucampia]

Merging this with only one approval as it is a fairly simple fix and includes extensive testing.