[pet] [vic20] [c64] [c16] [cplus4] [c128] [x16] [m65]
The declare keyword forward-declares a subroutine or function.
DECLARE SUB <name> ([arg1 AS <type> [, arg2 AS <type>, ... ]]) [OVERRIDE] [STATIC] [SHARED] DECLARE FUNCTION <name> AS <type> ([arg1 AS <type> [, arg2 AS <type>, ... ]]) [OVERRIDE] [STATIC] [SHARED]
Read more about forward declaration here.