<#assign x = pp.newWritableSequence(stuff)>
<@pp.add seq=x value='C' />
<#assign y = pp.newWritableSequence(x)>
<@pp.add seq=y value='D' />
<#assign z = pp.newWritableSequence(['X', 'Y'])>
<@pp.add seq=z value='Z' />

stuff:
<@listSequence stuff />

x:
<@listSequence x />

y:
<@listSequence y />

z:
<@listSequence z />

<#macro listSequence seq>
  <#list seq as i>
  - ${i}
  </#list>
</#macro>