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Let M be a totally orientable graph manifold with characteristic submanifold T and
let M = V ∪SW be a Heegaard splitting. We prove that S is standard. In particular,
S is the amalgamation of strongly irreducible Heegaard splittings. The splitting
surfaces Si of these strongly irreducible Heegaard splittings have the property that
for each vertex manifold N of M, Si∩N is either horizontal, pseudohorizontal,
vertical or pseudovertical.
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