10.6.1 GOSUB and RETURN Statements
  The GOSUB statement unconditionally transfers control to
  a line in a subroutine. The last statement in a subroutine
  is a RETURN statement, which returns control to the first
  statement after the calling GOSUB. A subroutine can con-
  tain more than one RETURN statement so you can return
  control conditionally, depending on a specified condition.

  The following example first assigns a value of 5 to the vari-
  able A , then transfers control to the subroutine labeled Times_
  two . This subroutine replaces the value of A with A multi-
  plied by 2. The subroutine's RETURN statement transfers
  control to the first PRINT statement, which displays the
  changed value. The program calls the subroutine two more
  times, with different values for A . Each time, the RETURN
  transfers control to the statement immediately following the
  corresponding GOSUB.
  EXAMPLE: Click to display example.

  Output
  10
  30
  50
  Note that BASIC signals ``RETURN without GOSUB'' if
  it encounters a RETURN statement without first having
  encountered a GOSUB or ON GOSUB statement.