  We present a recursive rule for the symbol of perturbative contributions to
the vacuum wave function of a conformally coupled scalar in FRW cosmologies.
The rule applies exactly for a class of interactions and cosmologies, which
contains {\lambda}{\phi}^3 in dS_4, a case of particular relevance as a source
of building blocks for inflationary correlators. We use the rule to efficiently
reproduce the tree-level four-point contribution and present novel computations
of the bubble integrand and the tree-level five-point contribution. Our results
apply equally well to the computation of Witten diagrams in Euclidean AdS.
