20.1.3 Passing Parameters by Value
  When you pass a parameter by value, you pass a copy of
  the parameter value to the routine instead of passing its ad-
  dress. Because the actual value of the parameter is passed,
  the routine does not have access to the storage location of the
  parameter; therefore, any changes that you make to the pa-
  rameter value in the routine do not affect the value of that
  parameter in the calling routine.