[From string (line 10) ]                                                               
                                                                                       
@startuml                                                                              
!unquoted function DRAW($x) return %set_variable_value($x, 1)                          
                                                                                       
!function addBox($part, $box, $colour)                                                 
    !if %variable_exists($part)                                                        
        box "$box" #$colour                                                            
            participant "xXXX" as xx                                                   
        end box                                                                        
    !endif                                                                             
!endfunction                                                                           
^^^^^                                                                                  
 This function does not have any !return directive. Declare it as a procedure instead ?