Find[x; xs].
Found[x].
NotFound[].

&Setup[];
&OptimizeFor[];

&Setup[].
---
\x: A["OptimizeFor"; \x];
E[&RecurseR[]];

&OptimizeFor[].
===
A["OptimizeFor"; r'
for (\i in \iter) {
  ...\in
}
'] &Env[] &Contains[x: r'\x[[\i]] <- \_']: r'
if (is.data.frame(\x)) {
  \y <- as.list(\x)

  for (\i in \iter) {
    ...\in
  }

  \x <- as.data.frame(\y)
}
' &Subst[old: r'\x[[\i]] <- \expr'; new: r'\y[[\i]] <- \expr'];
E[&RecurseR[]];

&Env[i; x; y].
---
\i: R_Symbol[\];
\x: R_Symbol[\];
r'y': \y;

&Contains[in; x].
---
Find[#Flush[\x]; \in] &Find[]: Found[\x];

&Find[].
===
Find[\; Nil[]]: NotFound[];
Find[\x; Cons[\y; \]] &IsTrue[x: #Any_Matches[\x; \y]]: Found[\y];
Find[\x; Cons[\; \xs]]: Find[\x; \xs];

&SubstIn[in; old; new].
---
\in: \in &Subst[];
E[Cons[\fst; \rst]]: Cons[E[\fst]; E[\rst]];
E[R_Call[R_Symbol["{"]; \exprs]]: R_Call[R_Symbol["{"]; E[\exprs]];
E[R_PairList[\exprs]]: R_PairList[E[\exprs]];
E[&RecurseR[]];

&Subst[old; new].
===
\old: \new;
E[&RecurseR[]];

&IsTrue[x].
---
\x: True[];

&RecurseR[].
---
E[R_Call[\fun; \args]]: R_Call[E[\fun]; #Base_Map[E[Hole[1]]; \args]];
E[R_PairList[\elems]]: R_PairList[#Base_Map[E[Hole[1]]; \elems]];