#### paper-column-interface

`Paper_column` objects form the top-most X-parents for items. The are two types of columns: musical columns, where are attached to, and non-musical columns, where bar-lines, clefs etc. are attached to. The spacing engine determines the X-positions of these objects.

They are numbered, the first (leftmost) is column 0. Numbering happens before line-breaking, and columns are not renumbered after line breaking. Since many columns go unused, you should only use the rank field to get ordering information. Two adjacent columns may have non-adjacent numbers.

##### User settable properties:

`between-cols` (pair)
  Where to attach a loose column to
`between-system-string` (string)
  string to dump between two systems. Useful for forcing page breaks.
`shortest-playing-duration` (moment)
  The duration of the shortest playing here.
`shortest-starter-duration` (moment)
  The duration of the shortest note that starts here.
`when` (moment)
  Global time step associated with this column happen?

##### Internal properties:

`bounded-by-me` (list of grobs)
  list of spanners that have this column as start/begin point. Only columns that have grobs or act as bounds are spaced.

This grob interface is used in the following graphical objects: NonMusicalPaperColumn and PaperColumn

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