code: public static Set<String> findPrefixesForMandatoryImportedVariables( Instance instance ) {
  		Set<String> result = new HashSet<> ();
  
  		for( ImportedVariable var : ComponentHelpers.findAllImportedVariables( instance.getComponent()).values()) {
  			if( ! var.isOptional())
  				result.add( VariableHelpers.parseVariableName( var.getName()).getKey());
  		}
  
  		return result;
  	}
docs: Finds the component and facet names that prefix the variables an instance requires.
  <p>
  Only the mandatory variables are returned. Optional imports are not considered by this method.
  </p>
  
  @param instance an instance
  @return a non-null set with all the component and facet names this instance imports