I/O

Basic input, output, store and read, copy and paste is all done with XML.
Example below.
<selection>
  <!--
      user: wolf
      time: 2020-12-04 14:30:55
  -->
  <elements>
    <line id="line-5" width="2.0" outline="black" smooth="0" constraint="">
      <dot id="dot-18" x="10.0" y="250.0" />
      <dot id="dot-19" x="300.0" y="200.0" />
    </line>
    <group id="group-0" width="" fill="" outline="" constraint="">
      <fill id="fill-0" fill="white" outline="" width="" constraint="">
        <line id="line-1" width="1.0" outline="black" smooth="0" constraint="">
          <dot id="dot-4" x="100.0" y="100.0" />
          <dot id="dot-5" x="200.0" y="100.0" />
        </line>
        <line id="line-2" width="5.0" outline="red" smooth="0" constraint="">
          <dot id="dot-5" x="200.0" y="100.0" />
          <dot id="dot-6" x="150.0" y="170.0" />
        </line>
      </fill>
      <line id="line-0" width="5.0" outline="black" smooth="0" constraint="">
        <dot id="dot-1" x="100.0" y="50.0" />
        <dot id="dot-2" x="150.0" y="120.0" />
      </line>
    </group>
    <fill id="fill-1" fill="white" outline="" width="" constraint="">
      <line id="line-3" width="2.0" outline="black" smooth="0" constraint="">
        <dot id="dot-10" x="10.0" y="10.0" />
        <dot id="dot-13" x="10.0" y="100.0" />
      </line>
      <line id="line-4" width="2.0" outline="black" smooth="0" constraint="">
        <dot id="dot-13" x="10.0" y="100.0" />
        <dot id="dot-14" x="50.0" y="50.0" />
      </line>
    </fill>
  </elements>
  <stack>
    <item ref="line-5" />
    <item ref="fill-0" />
    <item ref="line-1" />
    <item ref="line-2" />
    <item ref="line-0" />
    <item ref="fill-1" />
    <item ref="line-3" />
    <item ref="line-4" />
  </stack>
</selection>