# File lib/ruport/data/table.rb, line 494
    def replace_column(old_col,new_col=nil,&block)
      if new_col
        add_column(new_col,:after => old_col,&block)
        remove_column(old_col)
      else
        each { |r| r[old_col] = yield(r) }
      end
    end