20.1.1 Passing Parameters by Reference
  When you pass a parameter by reference, BASIC passes the
  address at which the actual parameter value is stored. In
  other words, your routine has access to the parameter's stor-
  age address; therefore, you can manipulate and change the
  value of this parameter. Any changes that you make to the
  value of the parameter in your routine are reflected in the
  calling routine as well.