Implementation of Symmetric Distance Function "Delta" as proposed in [Heyninck 2020]
Adapted from class "DalalDistance" for the use of Three Valued Logic

This class models an indecision operator for 3-valued propositional logic as proposed in [Heyninck 2020]
Indecision(a) is true, if formula a is undecided
Indecision(a) is false, if formula a is true or false
Adapted from the class "Negation"