@Override
    public BooleanExpression not() {
        if (opMixin.getOperator() == Ops.NOT && opMixin.getArg(0) instanceof BooleanExpression) {
            return (BooleanExpression) opMixin.getArg(0);
        } else {
            return super.not();
        }
    }
{@code BooleanOperation} represents boolean operations

@author tiwe
querydsl-core/src/main/java/com/querydsl/core/types/dsl/BooleanOperation.java