Flag of foliations

See @feigin1975characteristic.

Definition
Given a manifold $M$, a flag of foliations of codimensions $q_k\le q_{k-1}\le \cdots \le q_1$ is a sequence of foliations ${\mathcal{F}}_1,\dots ,{\mathcal{F}}_k$ such that for $i=1,\dots ,k$ the codimension of ${\mathcal{F}}_i$ is $q_i$ and for $i\le j$ the leaves of ${\mathcal{F}}_i$ are contained in those of ${\mathcal{F}}_j$.
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