Add relative address and at functions

doc/teapot.lyx

@@ -5393,20 +5393,8 @@ l
 \emph default
 ) 
 \series default
-returns the value of the cell at position 
-\emph on
-x
-\emph default
-, 
-\emph on
-y
-\emph default
-, 
-\emph on
-z
-\emph default
-.
- If any of 
+returns the value of the cell at the specified location.
+ In the first form, if any of 
 \emph on
 x
 \emph default
@@ -5465,9 +5453,30 @@ y
 
 z
 \emph default
-]]]) 
+]]])
+\begin_inset space ~
+\end_inset
+
+|
+\begin_inset space ~
+\end_inset
+
+
 \series default
-returns a pointer to the cell at location 
+&
+\series medium
+(location
+\emph on
+
+\begin_inset space ~
+\end_inset
+
+l
+\emph default
+) 
+\series default
+returns the specified location.
+ In the first form, if any of 
 \emph on
 x
 \emph default
@@ -5475,30 +5484,65 @@ x
 \emph on
 y
 \emph default
-, 
+ or 
 \emph on
 z
 \emph default
-.
- If 
-\emph on
-z
-\emph default
- is omitted, the 
-\emph on
-z
-\emph default
- position of the current cell is used.
- If 
-\emph on
-y
-\emph default
- is missing as well, the 
-\emph on
-y
-\emph default
- position (row) of the cell is used.
- 
+ is omitted, the coordinate of the cell is used.
+ The second form is in fact a no-op, but it is allowed for convenience
+\end_layout
+
+\begin_layout Description
+R() Exactly the same as 
+\series bold
+@
+\series default
+, except the coordinates of the argument are interpreted as an offset to
+ the current location (rather than as absolute coordinates in the sheet);
+ think 
+\begin_inset Quotes eld
+\end_inset
+
+R
+\begin_inset Quotes erd
+\end_inset
+
+ for 
+\begin_inset Quotes eld
+\end_inset
+
+relative.
+\begin_inset Quotes erd
+\end_inset
+
+ Thus R(-1) returns the value of the cell immediately to the left of this
+ one.
+\end_layout
+
+\begin_layout Description
+D() Exactly the same as 
+\series bold
+&
+\series default
+, except the coordinates of the argument are added to the current location.
+ Think 
+\begin_inset Quotes eld
+\end_inset
+
+D
+\begin_inset Quotes erd
+\end_inset
+
+ for 
+\begin_inset Quotes eld
+\end_inset
+
+displaced (by).
+\begin_inset Quotes erd
+\end_inset
+
+ Thus, D(-1) returns the location of the cell immediately to the left of
+ this one.
 \end_layout
 
 \begin_layout Description