Implemented:
Only simple tests for globally defined elements according to the rules a, b and c.

Implemented in other sections:
- Tests for the rule d are in the section 8
- Tests for the rule e are in the section 7.7
- Tests for nillable elements are in the section 7.1.11
- Tests for local elements are in the section 5.2.2 (naming) and 7.6
- Tests for minOccurs and maxOccurs are in the section 7.1.4