SPARQL evaluation semantics

Hello,

In section 12.5 the SPARQL spec. defines an evaluation semantics using an 
operation eval. The result of this operation seems to be a multiset of 
solution mappings and the 2nd operator is said to be a graph pattern. The 
given definitions contain symbols like LeftJoin. The meaning of these symbols 
confuses me. Do they represent graph patterns or algebra operators? Or, do 
you use the same symbol for both - graph patterns in the left part of the 
equation and algebra operators in the right?

Thanks,
Olaf