A true root release after the prefix (0,1,9): the word (0,1,9,3,54,27)

A tree witness is a function $\theta(k,r)\in\{0,1\}$ on the nodes $(k,r)$, $0\le r<2^k$, of the binary tree of residues. For distinct integers $x,y$ with $k=\nu_2(y-x)$, put $x\prec_\theta y$ iff $\mathrm{bit}_k(x)=\theta(k,\,x\bmod 2^k)$. A finite word $Q$ of integers is $\theta$-safe if the sequence "the letters of $Q$ in order, then all remaining integers in $\prec_\theta$-order" contains no monotone $4$-term arithmetic progression. An in-order pair $(a,b)$ of $Q$ ($a$ occurs before $b$) is open if $2b-a\notin Q$ and $3b-2a\notin Q$; it lies at the node $(k,\,a\bmod 2^k)$, $k=\nu_2(b-a)$, and forces $\theta$ there to equal $\mathrm{bit}_k(b)$. A root pair is one with $b-a$ odd (node $(0,0)$); $Q$ is a true root release if every open root pair has even second entry, i.e. no open pair forces the value $1$ at the root. The six-letter word

is tree-safe (with the witness $\theta(k,r)=1$ for $k\ge1$ and $\theta(0,0)=0$) and is a true root release: its open root pairs $(1,54),(9,54),(3,54)$ all have even second entry. Hence the root can be released after the three-letter prefix $(0,1,9)$. Here the forced $3$-chains of the odd letters $1$ and $9$ merge ($1\to3\to9\to27$), so a single grounded letter $27$ (preceded by $54$) suffices; for a general odd third letter $x$ the chains of $1$ and $x$ are disjoint and at least two grounded letters are needed (Erdős Problem 195, tree-witness approach).

Formalization note: treeLt θ is from Definitions.Def_Erdos195_tree; Safe, bitOne are from Definitions.Def_Erdos195_defs. Positions are List.idxOf; "starts with 0" is Q.head? = some 0; the release hypothesis is stated as: every open in-order pair $(a,b)$ with $(b-a) \bmod 2 = 1$ has $b \bmod 2 = 0$.