A formal specification can describe software models which are di±cult to program. Transformational methods based on fold/unfold strategies have been proposed to palliate this problem. The objective of applying transfor- mations is to filter out a new version of the specification where recursion may be introduced by a folding step. Among many problems, the "eureka" about when and how to define a new predicate is di±cult to find automatically. We propose a new version of the folding rule which decides automatically how to introduce new predicates in a specification. Our method is based on finding similarities between formulas represented as parsing trees and it constitutes an assistance to the complex problem of deriving recursive specifications from non recursive ones.