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\section{Introduction}   Observations with the Herschel satellite have revealed that the backbone of molecular clouds is a compex network of connecting and interacting filaments \citep[e.g.][]{2010A&A...518L.102A,2010A&A...518L.100M,2011A&A...529L...6A,2012A&A...540L..11S}. The dynamics of this web of dense molecular gas not only determines determine  the evolution and stability of molecular clouds, it but  also regulates regulate  their condensation into stars. Molecular cloud cores and single low-mass stars are almost always found in filaments, often aligned like pearls on a string \citep{2002ApJ...578..914H,2008ApJ...672..410L,2010A&A...518L.102A}. This can be interpreted as a result of the gravitational instability of a supercritical filamentary section \citep{1997ApJ...480..681I,2011A&A...533A..34H}. Where filaments intersect, more massive hubs form \citep{2009ApJ...700.1609M,2013ApJ...764..140M} that later on could become the progenitors of star clusters. \citet{2013ApJ...764..140M} \citep[see also e.g.][]{2007ApJ...666..982E} demonstrated that the fraction of molecular gas in the denst dense  molecular web is of order 10 percent of the total gas mass. Interestingly, the mass fraction of protostars to dense molecular gas is a constant of the same order. That is, the more stars there are, the more dense gas is available. \citet{2013ApJ...773...48B} showed that this requires new filamentary segments to continuously form from the diffuse intra-filament medium on a gravitational collapse timescale. Classically, gas filaments have been treated as cylinders of gas in hydrostatic equilibrium \citep{1964ApJ...140.1529O,2012A&A...542A..77F,2013A&A...558A..27R}. It is not clear, however, how such quiescent structures could form in the turbulent environment of a molecular cloud \citep[e.g.][]{2010A&A...520A..17K,2013ApJ...769..115H}. In addition, millimeter line studies indicate that filaments have intrinsic, super-thermal linewidths \citep{2013A&A...553A.119A}.  More recently recently,  observations by \citet{2013A&A...554A..55H}have  revealed that filaments are often compact bundles of thin spaghetti-like subfilaments. They presented observations of a prominent filamentary feature in Taurus, dominated by the L1495 cloud \citep{1962ApJS....7....1L} and the several dark patches \citep{1927cdos.book.....B}. Hacar et al. observed the region in the moderate density tracer C$^{18}$O, obtaining spectra along a $\sim 3$ pc length of the region. Analysing the filament in the resultant position--position--velocity space, they found the intriguing result that the gas along the ridge is organized in velocity-coherent filamentary structures, with typical lengths $\sim0.5$ pc. Each filament is internally subsonic or transsonic, though the collection of filaments is characterised by a mildly supersonic interfilamentary dispersion of $\sim 0.5$ km s$^{-1}$. Multiple velocity components along a single line of sight have also been observed in Serpens South \citep{2013ApJ...778...34T}; this feature may be common one in young star forming sites.  The complexity of Hacar et al.'s position--position--velocity data invites theoretical and numerical exploration. In particular the apparent organization of filaments into bundles tends to bring to mind magnetic fields or other relatively complex physics.