fact: Definition Fgraph m0 n0 := [set m1 | inl m1 \in subgraph (tanner_rel H) (inl m0) (inr n0)].
imports: mathcomp(all_ssreflect ssralg fingroup finalg perm zmodp) mathcomp(matrix) bigop_ext ssralg_ext f2 subgraph_partition