Signatures, formulas and trees
The next important concept is the notation of the signature. The signature describes the interface to the scoring function needed in our algorithm. We can derive the signature for our current example by simply interpreting each of the candidate trees' node labels as a function declaration:
number:	A	   -> Ans
minus:	Ans x Ans  -> Ans
times:	Ans x Ans  -> Ans
plus:	Ans x Ans  -> Ans
div:	Ans x Ans  -> Ans
The symbol Ans in the abstract result domain. In the following, Σ denotes the signature, T_Σ the set of trees over the signature Σ.
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