static Boolean opNot(Boolean op) {
    // When people forget to quote a string, op1/op2 is null.
    // For example, select * from some_table where not ds > 2012-12-1 .
    if (op != null) {
      if (op.equals(Boolean.TRUE)) {
        return Boolean.FALSE;
      }
      if (op.equals(Boolean.FALSE)) {
        return Boolean.TRUE;
      }
    }
    return null;
  }
Expression processor factory for partition condition removing. Each processor tries to
calculate a result vector from its children's result vectors.
Each element is the result for one of the pruned partitions.
It also generates node by Modifying expr trees with partition conditions removed
ql/src/java/org/apache/hadoop/hive/ql/optimizer/pcr/PcrExprProcFactory.java