Flattens a list.
[lf,ind] = aplat(l,r)
:l a list : :r an optional flat list : :lf a flat list (a single hierachical level) : :ind a list, each entry give the path of the corresponding lf entry
in the original list
:
Creates a flat list, built with the initial l list leaves and if given prepended by the r list entries
[lf,ind]=aplat(`list`_(1,2,`list`_([3,1],'xxx',`list`_([3,2,1]))))