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Adding definition and example for specification, specifiable classes |
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Formally, a specification for a set of combinatorial classes <math>(\mathcal A_1,\dots,\mathcal A_r)</math> is a set of <math>r</math> equations <math>\mathcal A_i=\Phi_i(\mathcal A_1,\dots,\mathcal A_r)</math>, where <math>\Phi_i</math> is an expression, whose atoms are <math>\mathcal E,\mathcal Z</math> and the <math>\mathcal A_i</math>'s, and whose operators are the elementary constructions listed above.
A class of combinatorial structures is said to be ''
For example, the set of trees whose leaves's depth is even (respectively, odd) can be defined using the specification with two classes <math>\mathcal A_{even}</math> and <math>\mathcal A_{odd}</math>. Those clasess should satisfy the equation <math>\mathcal A_{odd}=\mathcal Z\times \mathrm{Seq}_{\ge1}\mathcal A_{even}</math> and <math>\mathcal A_{even}=\mathcal Z\times \mathrm{Seq}\mathcal A_{odd}</math>.
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