def insert_before( child1, child2 )
      if child1.kind_of? String
        child1 = XPath.first( self, child1 )
        child1.parent.insert_before child1, child2
      else
        ind = index(child1)
        child2.parent.delete(child2) if child2.parent
        @children[ind,0] = child2
        child2.parent = self
      end
      self
    end
def insert_before( child1, child2 )
      if child1.kind_of? String
        child1 = XPath.first( self, child1 )
        child1.parent.insert_before child1, child2
      else
        ind = index(child1)
        child2.parent.delete(child2) if child2.parent
        @children[ind,0] = child2
        child2.parent = self
      end
      self
    end
Inserts an child before another child
 @param child1 this is either an xpath or an Element.  If an Element,
 child2 will be inserted before child1 in the child list of the parent.
 If an xpath, child2 will be inserted before the first child to match
 the xpath.
 @param child2 the child to insert
 @return the parent (self)