code: def footer=(footer)
           if footer.type == FooterNode::UNIVERSAL
              @footers[0] = footer
           elsif footer.type == FooterNode::LEFT_PAGE
              @footers[1] = footer
           elsif footer.type == FooterNode::RIGHT_PAGE
              @footers[2] = footer
           elsif footer.type == FooterNode::FIRST_PAGE
              @footers[3] = footer
           end
        end
docs: This method assigns a new footer to a document. A Document object can
   have up to four footers - a default footer, a footer for left pages, a
   footer for right pages and a footer for the first page. The method
   checks the footer type and stores it appropriately.
  
   ==== Parameters
   footer::  A reference to the footer object to be stored. Existing footer
             objects are overwritten.