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The authors of the paper introduced a formatting error that led to some characters in Definition 5 being written in the incorrect font case. In turn, this changed the meaning of the definition which rendered the work inaccurate. Unfortunately, the errors were overlooked during the proofing stage of the publication process for which the authors apologise.
Definition 5 should read:
Instead of 'Let x be a universal set', it should read 'Let X be a universal set'
And
Instead of 'x ∈ x', it should read 'x ∈ X'