class REXML::Elements

elements.index(ele_3) # => -1
elements.index(ele_4) # => 3
elements.delete(ele_3)
elements.index(ele_4) # => 4
ele_1, ele_2, ele_3, ele_4 = *elements
elements = d.root.elements
d = REXML::Document.new(xml_string)

otherwise, returns -1:
Returns the 1-based index of the given +element+, if found;

index(element)
:call-seq:

def index element
  rv = 0
  found = @element.find do |child|
    child.kind_of? Element and
    (rv += 1) and
    child == element
  end
  return rv if found == element
  return -1
end