# Prefer to fill in a square with only one choice.
sp {prefer*fill*in*single*number
   (state <s> ^operator <o1> +)
   (<o1> ^name fill-in
         ^square <sq1>
         ^value <int1>)
   (<sq1> ^row <row> ^column <col>)
 - { (<s> ^operator <o2> +)
     (<o2> ^name fill-in
           ^square <sq1>
           ^value { <> <int1> } ) }
-->
   (<s> ^operator <o1> > = )
   (write (crlf) | Only one choice: | <int1> | for row: | <row> | column: | <col> )}