GOSUB expression

   Synopsis:
      Calls a pseudo-subroutine

   Notes:
      A pseudo-subroutine is one called by a GOSUB statement in an Axbasic
         script with primitive line numbers. (A 'real' subroutine is one in an
         Axbasic script without line numbers, which is declared with a SUB
         statement.)
      The expression must evaluate to a line number or an error will occur.
         Complex expressions like '(x + 10) / 2' can be used as well as simple
         line numbers like 100.
      Execution jumps to the specified line number, but when the first RETURN
         statement is encountered, execution returns to the first statement
         after the initial GOSUB statement.
      In the example below, the lines are executed in the order 10, 20, 30, 100,
         110, 120, 40, 50, 60.
      See also the help for IF and ON.

   Availability:
      GOSUB is only available in scripts with primitive line numbers.

   Examples:
      10 PRINT "Hello world!"
      20 PRINT "We will now jump into the pseudo-subroutine at line 100"
      30 GOSUB 100
      40 PRINT "We have left the pseudo-subroutine"
      50 PRINT "Goodbye cruel world!"
      60 END
      100 PRINT "You called the pseudo-subroutine"
      110 PRINT "Now we will go back to line 40"
      120 RETURN
