Program Unit ☆
The main program and external procedures are called Program units.
Declarations by OPTION or DECLARE statements, the data sequence provided by DATA statements, variable names，channel numbers except zero and so on are independent for each program unit.
OPTION ANGLE, OPTION BASE must be written within each program unit, but each program unit can be set differently,
DECLARE EXTERNAL FUNCTION must be written within every program unit that uses the function,
and the data written in another program unit can not be read.