def previous_element
      element = previous_sibling
      element = element.previous_sibling until element.nil? or element.kind_of? Element
      return element
    end
def previous_element
      element = previous_sibling
      element = element.previous_sibling until element.nil? or element.kind_of? Element
      return element
    end
Returns the previous sibling that is an element, or nil if there is
 no Element sibling prior to this one
  doc = Document.new '<a><b/>text<c/></a>'
  doc.root.elements['c'].previous_element          #-> <b/>
  doc.root.elements['b'].previous_element          #-> nil