svg.rb

class SVG
  def self.rect(width, height, x, y)
    "<rect width=\"#{width}\" height=\"#{height}\" x=\"#{x}\" y=\"#{y}\"/>"
  end

  def self.build(width, height, &block)
    rect_output = instance_eval(&block)

    "<svg width=\"#{width}\" height=\"#{height}\" xmlns=\"http://www.w3.org/2000/svg\">\n" +
    "    #{rect_output}\n" +
    "</svg>"
  end
end

output = SVG.build(300, 200) do
  rect(200, 100, 10, 10)
end

puts output

# Выведет то, что и запрашивалось:
#
#<svg width="300" height="200" xmlns="http://www.w3.org/2000/svg">
#    <rect width="200" height="100" x="10" y="10"/>
#</svg>