with $O$, which is impossible since $G_m \approx O(a)$.

b\) By Lemma 5.1, we have $t \p_C (f)t_1...t_r$, and, by a), $t=(f)t_1...t_r$. Therefore $\G
\v_{C2_O} (f)t_1...t_r:O(a)$.

-   - If $f=x_i$ $1 \leq i \leq m$, then $r=0$, $t=x_i$, and $O(a_i) \approx O(a)$, then $a_i
    \approx a$.

-   - If $f=y_j$ $1 \leq j \leq k$, then $A_j \lhd_O F_1 \f G_1$, $G'_k \lhd_O F_{k+1} \f
    G_{k+1}$ $1 \leq k \leq r-1$, $G_r \approx O(a)$, $G_k \approx G'_k$ $1 \leq k \leq r$, and $\G\v_{C2_O} t_k:F_k$ $1 \leq k \leq r$. Since $A_j$ is not ending with $O$, then $G_k$ $1 \leq
    k \leq r$ is not ending with $O$, which is impossible since $Cr \approx O(a)$. $\Box$

Let $V$ be the set of variables of $\l C$-calculus.\
Let $P$ be an infinite set of constants called stack constants [^2].\
We define a set of $\l C$-terms $\L CP$ by :

-   - If $x \in V$, then $x \in \L CP$ ;

-   - If $t \in \L CP$, and $x \in V$, then $\l xt \in \L CP$ ;

-   - If $t \in \L CP$, and $u \in \L CP \bigcup P$, then $(t)u \in \L CP$.

In 