operator: ==

*    A term:

     Var1 == Var2

     ...will be considered true if Var1 and Var2 are unified to the same value

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operator: \+ - negation

*    \+(P) is true if P fails (cannot be proven)

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variable: _ (underscore) - don't care

...we don't care what variable _ unifies with,
   and we don't care if several _'s unify to the same or different things;

...like a wild card;

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