@code{"%.3f"} all the cells using printer @samp{foo} will be reprinted
accordingly.
+When you define a printer function with a lambda expression taking one
+argument, please take care that the returned value is a string, or a
+list containing a string, even when the input argument has an
+unexpected value. Here is an example:
+
+@example
+(lambda (val)
+ (cond
+ ((null val) "")
+ ((and (numberp val) (>= val 0)) (format "%.1f" val))
+ (t (ses-center-span (format "%S" val) ?#))))
+@end example
+
+This example will:
+@itemize
+@item
+When the cell is empty (ie.@: when @code{val} is @code{nil}), print an
+empty string @code{""}
+@item
+When the cell value is a non negative number, format the the value in
+fixed-point notation with one decimal after point
+@item
+Otherwise, handle the value as erroneous by printing it as an
+s-expression (using @code{prin1}), centered and surrounded by @code{#}
+filling.
+@end itemize
+
+
+
+
@node Clearing cells
@section Clearing cells
@cindex clearing commands