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<p>both of which are part of the formal language.<br/>
* In some cases an <a href="page.php?w=Mathematical_induction">inductive system</a>, used to derive a proof by first establishing a simple case, then generalizing it.</p>

<p>A formal system is said to be <a href="page.php?w=Recursive_set">recursive</a> (i.e. effective) or recursively enumerable if the set of axioms and the set of inference rules are <a href="page.php?w=decidable_set">decidable set</a>s or <a href="page.php?w=recursively_enumerable_set">semidecidable sets</a>, respectively.</p>

<p><big> Formal language </big></p><p>
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