18.2.2.1: Expand Syntax Forms: Flatten TripleCollection
18.2.2.3: Translate Property Path Expressions
18.2.2.4: Translate Property Path Patterns
18.2.2.5: Translate Basic Graph Patterns
TODO: In the ast, a group with a single BGP in it is a single object. (TODO: not anymore)
18.2.2.1: Expand Syntax Forms: Flatten TripleCollection 18.2.2.3: Translate Property Path Expressions 18.2.2.4: Translate Property Path Patterns 18.2.2.5: Translate Basic Graph Patterns TODO: In the ast, a group with a single BGP in it is a single object. (TODO: not anymore)