I can Sum a column in Spreadsheet View

Summary:

In a large view, I click on a column header, and a pane opens up at the bottom of the sheet; in xs view, I long-press and an inset window opens up. It offers me functions I can perform on this column, such as Sum.

When I select Sum, does this attach as a permanent cell at the bottom of the Spreadsheet View? That may make sense -- it roughly matches expectations from spreadsheets -- but in that case we need a way to remove that as well, and need meta-Properties recording this decision.

For numeric columns only, but this is the camel's nose in the tent. Functions can declare themselves as relevant to a Type, probably through the beginnings of the function-signature mechanism, to be offered in Spreadsheet View like this.