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+title = "Recursive predicated expressions"
+author = "Yann Herklotz"
+tags = []
+categories = []
+backlinks = ["3d2b1", "3c3g2", "3c3g"]
+forwardlinks = ["2e1c6a", "3c3g2"]
+zettelid = "3c3g1"
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+
+These are expressions of the following type:
+
+$$ P ::= [p?, P] \ |\ e $$
+
+This means that one can either have a list of predicates, linked to more
+predicated expressions, or be an expression. The one difficulty with
+dealing with such an expression is that the comparison using a SAT
+solver becomes basically impossible. This is because one cannot perform
+hashing of the predicates, as described in `Ptree` hashing ([\#2e1c6a]).
+The hashing of the expressions is necessary to be able to pass the
+expressions to the SAT solver in the first place, and because one
+doesn't have pure expressions anymore, one cannot define the syntactic
+equality operator anymore to compare (and therefore hash) expressions.
+
+  [\#2e1c6a]: /zettel/2e1c6a