class REXML::Element

e['attr'] # => "VALUE"
e.add_attribute(a) # => attr='VALUE'
a = REXML::Attribute.new('attr', 'VALUE')
e['attr'] # => "value"
e.add_attribute(a) # => attr='value'
a = REXML::Attribute.new('attr', 'value')

adds the given attribute:
With only attribute object +attribute+ given,

e['attr'] # => "VALUE"
e.add_attribute('attr', 'VALUE') # => "VALUE"
e['attr'] # => "value"
e.add_attribute('attr', 'value') # => "value"
e = REXML::Element.new

adds the attribute created with that name and value:
With string argument +name+ and object +value+ are given,

by the same name.
Adds an attribute to this element, overwriting any existing attribute

add_attribute(attribute) -> attribute
add_attribute(name, value) -> value
:call-seq:

def add_attribute( key, value=nil )
  if key.kind_of? Attribute
    @attributes << key
  else
    @attributes[key] = value
  end
end