A change of a row in a Masu.Write:
index- its place in the write;id- the row's id (nil for a new row);row- the row, as the grid's source reads it now (nil for a new row; for:restore, the row as it was deleted);params- for:updateand:create, the values by the columns' fields, as strings, as the browser sends them (%{"total" => "11.50"}), which a changeset casts as it does a form's; an empty value is nil;keys- the keys of the columns the params are of;before- for:update, the value of each cell that the person saw, by the column's key.