Singleton pattern

Code:

macro __imp_ n {
        match =n,n \{
         display '__imp_' # `n,13,10
               extrn __imp_\#n :QWORD
         n equ __imp_\#n
    \}
    call [n]
}

How it works? First, MACRO invocation replaces "n" in the MATCH line to become: MATCH =Name,Name. Next, if Name is referenced to another value with EQU then the MATCH will resolve to EQU value - which should not be Name. Otherwise Name does = Name.

Note how several macro calls result in only the first MATCH block being executed and all from the preprocessor because MATCH resolves EQU. This pattern can be used to create a list of unique items without searching the list for existing entries. The EQU could be a LOCAL name to encapsulate the abstraction.