# Concepts

label
  A descriptive text in a cell that is not used for any calculations.
expression
  A string in a cell which evaluates to a (floating point) numeric result. The simplest expression is a number; more complicated expressions can be entered in Scheme syntax.

  The Scheme used in Siag has a few extensions to make it more useful in a spreadsheet. These new functions have been added:

  - (row) Returns the row number of the cell which is being computed.
  - (col) Returns the column number of the cell which is being computed.
  - (get-cell row col) Returns the contents of the cell with the given coordinates.
  - (sum start end) Returns the sum of all the cells in the rectangle between start and end.
grid
  The area where labels and expressions are displayed.
colnum
  The grey area where column numbers are displayed.
rownum
  The grey area where row numbers are displayed.
mark
  The "other" standard position in the document, which is used for commands which need two coordinates.
point or current cell
  The cell where labels and expressions are entered. The current cell can be moved using arrow keys, the mouse and other methods.
block
  The block is a rectangular area which is displayed in inverse. It is used for commands which operate on a whole range of cells.