 A related tradition was also quite influential on the background of the full abstraction problem, namely the characterizations of semantical notions like continuity and sequentiality inside syntactic models of the (untyped) \(\lambda\)-calculus based on Böhm trees (Barendregt 1984), mainly due to Lévy and Berry (see Berry et al. 1985 and Curien 1992) for accounts of the search for fully abstract models of PCF along this line).