<stx:transform version="1.0" 
  xmlns:gr="http://my_groups"
  xmlns:stx="http://stx.sourceforge.net/2002/ns"
  strip-space="yes" 
  pass-through="none">

  <stx:template match="/root">
    <a>
      <stx:call-procedure name="proc1"/>
    </a>
  </stx:template>

  <stx:procedure name="proc1">
    <b>
      <stx:call-procedure name="gr:proc2"/>
    </b>
  </stx:procedure>

  <stx:procedure name="gr:proc2">
    <c>
      <stx:call-procedure name="proc3" group="G1"/>
      <stx:call-procedure name="proc3"/>
    </c>
  </stx:procedure>

  <stx:procedure name="proc3">
    <e>OK</e>
  </stx:procedure>

  <stx:group name="G1">

    <stx:procedure name="proc3">
      <d>OK</d>
    </stx:procedure>

  </stx:group>
  
</stx:transform>