Does someone know what is meant (in the context of trees and Boolean algebras by Shelah) here on the page 8 right above Remark 1.5:
$$\{\langle\rangle\}\cup\{\langle\xi\rangle\otimes_{\zeta(*)}d\eta:\xi\in w \text{ and }\eta\in I_\xi\}$$ ? I do not know what $d$ means in this notation, nor can I find it in the text. All of the relevant notation (except for the questioned $d$) is given on the pages 8 and 9.