Let $k$ be a perfect field, $V$ be an irreducible smooth $k$-scheme and $W$ is an open dense subset of $V$. Let $F$ be the complement of $W$.
In this lemma, Morel states that there is an increasing sequence of reduced closed subschemes:
$$\emptyset= F_{-1}\subset F_0\subset\dots\subset F_d=F$$ such that each $k$-scheme $F_s-F_{s-1}$ is smooth and $\dim F_i=i$.
I have no ideal about this decomposition. How is this sequence deduced?
Could anyone help me?