Trees and gaps from a construction scheme
  We present natural constructions of trees and gaps using a quite general construction scheme. In particular, we solve a natural problem about $(\omega_1, \omega_1)$-gaps. As it is well known $(\omega_1, \omega_1)$-gaps can sometimes be filled in $\omega_1$-preserving forcing extensions of the set-theoretic universe. There are two natural conditions, dubbed $S$ and $T$ below, that guarantee the existence of such forcing extensions. The condition $T$ is a natural strengthening of the condition $S$ and was motivated by the numerous analogies between $(\omega_1,\omega_1)$-gaps and certain trees of height $\omega_1.$ It turns out that the condition $S$ is in fact equivalent to the existence of such forcing extensions but we show that the condition $T$ is strictly stronger by proving that it is consistent that there are fillable $(\omega_1, \omega_1)$-gaps (i.e., S-gaps) but no T-gaps. 
Fulgencio Lopez, Stevo Todorcevic
math.LO