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Line Identifiers
 
BASIC supports two types of line-identifiers: alphanumeric line labels
and line numbers.
 
    Alphanumeric line labels
        ■ Consist of 1 to 40 letters and digits.
        ■ Must start with a letter and end with a colon.
        ■ Are not case sensitive.
        ■ Cannot consist of BASIC keywords.
        ■ Can have blanks and tabs between the label and the colon.
        ■ The following are examples of valid alphanumeric line labels:
                ALPHA:
                ScreenSub:
                Alpha:
        ■ Special Notes:
             -  A GOTO statement is required when using alphanumeric line
                labels as objects in IF...THEN statements:
                    IF A = 10 THEN GOTO IncomeData
 
   Line numbers
        ■ Can range 0 to 65,529
        ■ The following are examples of valid line numbers:
                  1
                200
                300 PRINT "hello"     '300 is the line number.)
               6500
        ■ Special Notes:
             - Using 0 is not recommended because:
                    - error-trapping and trapping statements such as ON
                      ERROR and ON event) interpret the line number 0, to
                      mean that trapping is disabled (stopped)
                    - RESUME 0 continues execution on the line where the
                      error occurred, not at line number 0
             - Line numbers do not determine the order in which statements
               are executed. For example, BASIC executes statements in the
               following program in the order 100, 10, 5:
                    100 PRINT "The first line executed."
                     10 PRINT "The second line executed."
                      5 PRINT "The final line executed."
             - Some older BASICs, such as BASICA, expect the lines to be in
               numerical order: 5, 10, 100.
             - If you are trapping errors, the ERL function returns only the
               last line number located before the error.
 
Line Identifiers in General
    ■ They may begin in any column but must be first character in the line.
    ■ A line may have only one label.
    ■ BASIC does not require each line in a source program to have the
      same type of identifier.
    ■ You may mix alphanumeric labels and line numbers in the same program,
      and you may use alphanumeric labels as objects of any BASIC statement
      where line numbers are permitted except as the object of an IF...THEN
      statement.