code: public static boolean hasExpression(final Template template, final String expression) {
          final TemplateElement rootTreeNode = template.getRootTreeNode();
  
          return hasExpression(template, expression, rootTreeNode);
      }
docs: Determines whether exists a variable specified by the given expression
  in the specified template.
  
  @param template   the specified template
  @param expression the given expression, for example,
  "${aVariable}", "&lt;#list recentComments as comment&gt;"
  @return {@code true} if it exists, returns {@code false} otherwise