public void addToTemplate(TemplateNode child) throws StatementExecutionException {
    if (children.containsKey(child.getName())) {
      throw new StatementExecutionException("Duplicated child of node in template.");
    }
    children.put(child.getName(), child);
  }
The template instance constructed within session is SUGGESTED to be a flat measurement template,
which has no internal nodes inside a template.

<p>For example, template(s1, s2, s3) is a flat measurement template, while template2(GPS.x,
GPS.y, s1) is not.

<p>Tree-structured template, which is contrary to flat measurement template, may not be supported
in further version of IoTDB
iotdb-client/isession/src/main/java/org/apache/iotdb/isession/template/Template.java