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A NOT conversion visitor that will traverse the expression tree in a depth-first
manner with post-order handling. A status of negativity for a NOT expression is
recorded in the depth-first traversal before subtree traversal and reversed after
traversing the subtree. All "reversible" expressions is replaced by its negated
counterpart for negative negativity. Currently equality op, and its
non-equality counter part, all range comparisons, logical AND and OR are reversible.
Notably missing is the "is null" for lack of a "is not null" base expression;
UDF and regex are not reversed too for the same sake.
for (int i = 0; i < sucs.length; i++)
plan.connect(pred, pos.first+i, sucs[i], pos.second+i);
Begin traversing the graph.
if (successors == null) return;