Spill operator now works both as trailing `#` and ARRAYANCHOR function. `#` is converted to ARRAYANCHOR when writing. I do not think it is important to convert the other way when reading.
Documentation updates have started, but are a work in progress.
SINGLE function is implemented. I believe it works correctly when referring to a cell, but not when referring to a cell range. No attempt is yet made to convert leading `@` to and from SINGLE; I haven't figured out how to do so without interfering with `@` in structured references.
ISREF has problems. At least one of its tests was wrong, and many of those that were right were so accidentally. The code is changed, quite kludgily, so that almost all the tests are now deliberately correct. One very complicated test is incorrect; for now, I will skip it, and will open an issue when this PR is merged.
Implement SPILL for dynamic arrays. Calculating a dynamic array function will result in a SPILL error if it attempts to overlay a non-null cell which was not part of its previous calculation. Furthermore, it will set to null all cells which were part of its previous calculation but which are not part of the current one (i.e. one or both of the dimensions of the calculation is smaller than it had been); this should also apply for spills (whose result is reduced to 1*1).
Excel will stop you from changing the value in any cell in a dynamic array except the formula cell itself. I have not built this particular aspect into PhpSpreadsheet.
As usual, MS has taken some unusual steps here. If the result of a dynamic array calculation is #SPILL!, it will nevertheless be written to the xml as #VALUE!. It recognizes this situation by adding a new `vm` attribute to the cell, and expanding metadata.xml to recognize this.
A new optional parameter `$reduceArrays` is added to `toArray` and related functions. This will reduce a dynamic array to its first cell, which seems more useful than outputing it as an array (default).
oleibman