To include another sourcefile in your code, use the #import directive. You can also make a conditional import with the #importif directive.
#import "MyLibrary.asm" #importif STAND_ALONE "UpstartCode.asm" //<- Only imported if STAND_ALONE is defined
To ensure that a file (e.g. a library) is only imported once, place an #importonce inside the imported file
File1.asm: #importonce .print "This will only be printed once!" File2.asm: #import "File1.asm" // This will import File1 #import "File1.asm" // This will not import anything