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       # MATH introduce logical implication
[hear] (intro =>);

[hear] (define => /
         ? x /
         ? y /
         not /
         and (x) (not / y));

[hear] (=> (true) (true));

[hear] (not / => (true) (false));

[hear] (=> (false) (true));

[hear] (=> (false) (false));

[hear] (forall
          (? x /
           forall
            (? y /
             => (=> (x) (y)) (=> (not / y) (not / x)))));