Hledger.Write.Spreadsheet

• NoSpan means a single unmerged cell.

• Covered is a cell if it is part of a horizontally or vertically merged cell. We maintain these cells although they are ignored in HTML output. In contrast to that, FODS can store covered cells and allows to access the hidden cell content via formulas. CSV does not support merged cells and thus simply writes the content of covered cells. Maintaining Covered cells also simplifies transposing.

• SpanHorizontal n denotes the first cell in a row that is part of a merged cell. The merged cell contains n atomic cells, including the first one. That is SpanHorizontal 1 is actually like NoSpan. The content of this cell is shown as content of the merged cell.

• SpanVertical n starts a vertically merged cell.

The writer functions expect consistent data, that is, Covered cells must actually be part of a merged cell and merged cells must only cover Covered cells.

The same as Tab.Properties, but has Eq and Ord instances. We need those for storing NumLines in Sets.