Interface Minus

Since our graph translation is not really part of the spec, there is a MINUS edge case we need to consider with GRAPH ?g. If left and right of MINUS have disjoint variables, the whole left solution sequence must be kept. If GRAPH is defined outside of an operator (e.g. MINUS), then the spec says that evaluation of the operators must be done as union over the evaluation of that operator within each graph separately, and that the variable of ?g must only be bound after that evaluation. As such, MINUS will not be aware of this variable ?g, and the disjoint case will apply. The code below adds metadata to the operation so that engines can special-case this.