def lookup( value, stride )
      if value.is_a? Variable
        Lookup.new self, value, stride
      else
        Lookup.new @p.lookup( value, stride ), @index, @stride
      end
    end
def lookup( value, stride )
      if value.is_a? Variable
        Lookup.new self, value, stride
      else
        Lookup.new @p.lookup( value, stride ), @index, @stride
      end
    end
Lookup element of an array

 @param [Node] value Index of element.
 @param [Node] stride Stride for iterating over elements.

 @private