code: def format_column idx, format=nil, opts={}
        opts[:worksheet] = self
        res = case idx
              when Integer
                column = nil
                if format
                  column = Column.new(idx, format, opts)
                end
                @columns[idx] = column
              else
                idx.collect do |col| format_column col, format, opts end
              end
        shorten @columns
        res
      end
docs: Sets the default Format of the column at _idx_.
  
   _idx_ may be an Integer, or an Enumerable that iterates over a number of
   Integers.
  
   _format_ is a Format, or nil if you want to remove the Formatting at _idx_
  
   Returns an instance of Column if _idx_ is an Integer, an Array of Columns
   otherwise.