# File ai/csp/variable.rb, line 58
        def prune(level, value_index)
            if @sizes.length <= level
                last = @sizes[-1]
                (level).downto(@sizes.length) {|i|
                    @sizes[i] = last
                }
            end

            @pruned[value_index] = level
            @sizes[level] -= 1
        end