Graph isomorphy test is asymmetric

[MartinKl]

When two SDocumentGraphs a and b are distinct in an SSpan's coverage (b's span's coverage is a subset of a's span's coverage), the graph comparison via findDiffs(...) reveals to not be symmetric. The difference is not detected in a.findDiffs(b), but in b.findDiffs(a).

See also the following test snippet:

@Test
public void testGraphIsoTest() {
	String text = "This is a test. With spans.";
	SDocumentGraph a = SaltFactory.createSDocumentGraph();
	a.createTextualDS(text);
	a.tokenize();
	a.createSpan( a.getTokens().get(1), a.getTokens().get(2), a.getTokens().get(3) );
	a.setDocument( SaltFactory.createSDocument() );

	SDocumentGraph b = SaltFactory.createSDocumentGraph();
	b.createTextualDS(text);
	b.tokenize();
	b.createSpan( b.getTokens().get(1), b.getTokens().get(2) );
	b.setDocument( SaltFactory.createSDocument() );

	assertEquals(a.findDiffs(b).size(), b.findDiffs(a).size()); // fails, expects 0 and finds 1
}

The reported trace is:

java.lang.AssertionError: expected:<0> but was:<1>
	at org.junit.Assert.fail(Assert.java:88)
	at org.junit.Assert.failNotEquals(Assert.java:743)
	at org.junit.Assert.assertEquals(Assert.java:118)
	at org.junit.Assert.assertEquals(Assert.java:555)
	at org.junit.Assert.assertEquals(Assert.java:542)
	at org.corpus_tools.pepperModules.sgsTEIModules.SgsTEIImporterTest.testDummyTest(SgsTEIImporterTest.java:145)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)