code: public static Set<String> findPrefixesForExternalImports( AbstractApplication app ) {
  
  		Set<String> result = new TreeSet<> ();
  		for( Component c : ComponentHelpers.findAllComponents( app )) {
  			for( ImportedVariable var : ComponentHelpers.findAllImportedVariables( c ).values()) {
  				if( ! var.isExternal())
  					continue;
  
  				String prefix = VariableHelpers.parseVariableName( var.getName()).getKey();
  				result.add( prefix );
  			}
  		}
  
  		return result;
  	}
docs: Finds the prefixes of the external variables for a given application or template.
  <p>
  If this list is not empty, it means this application depends on
  other ones.
  </p>
  
  @param app an application or an application template
  @return a non-null application