class String

def pathmap_partial(n)

directories from the back end (right hand side) if +n+ is negative.
front end (left hand side) if +n+ is positive. Include |+n+|
Extract a partial path from the path. Include +n+ directories from the 
def pathmap_partial(n)
  dirs = File.dirname(self).pathmap_explode
  partial_dirs =
    if n > 0
      dirs[0...n]
    elsif n < 0
      dirs.reverse[0...-n].reverse
    else
      "."
    end
  File.join(partial_dirs)
end