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Internal Subprogram ☆
A subprogram describes a definite procedure as an object.
An internal subprogram is a subprogram written within a program unit, begins with a SUB line and ends with a END-SUB line.
A SUB line describes its name and parameters. The parameters are written separated by commas and enclosed in parentheses if any.
The rule for naming a subprogram is the same as that for a numeric variable.
Use a CALL statement to execute a subprogram. To execute a subprogram that has parameters, write the arguments enclosed in parentheses as in function reference.
Example

    100 OPTION ANGLE DEGREES
    110 SUB circle(a,b,r)
    120    FOR t=0 TO 360
    130       PLOT LINES: a+r*COS(t),b+r*SIN(t);
    140    NEXT t
    150    PLOT LINES
    160 END SUB
    170 SET WINDOW -4,4,-4,4
    180 CALL circle(1,-1,2)
    190 CALL circle(-1,2,1)
    200 END

All variables except parameters are common with the program unit.
When the control comes to the SUB-line, it jumps to the next line to the END-SUB line.
If a CALL statement is executed , the control jumps to the SUB-line, and if it reaches the END-SUB-line, it returns to the next line to the CALL-line.