L_MASK    equ 011100000000y            ; mask to isolate language bits
L_OPTLINK equ 011100000000y            ; setting for OptLink calling convention
VerifyCallBack macro ProcName
  if (((OPATTR (ProcName)) and L_MASK) ne L_OPTLINK)
     .err <Call-back routine must have OptLink linkage>
  endif
endm