13.2 Declaring Subprograms and Parameters
  You declare a subprogram by naming it in an EXTERNAL
  statement in the calling program. You may also declare the
  data type of each parameter. If the subprogram is a function,
  the EXTERNAL statement also lets you specify the data type
  of the returned value.

  The following statements are subprogram declarations using
  the EXTERNAL statement:
  EXTERNAL SUB my_sub (LONG, STRING)
  EXTERNAL GFLOAT FUNCTION my_func (GFLOAT, LONG, GFLOAT)
  EXTERNAL REAL FUNCTION determinant (LONG DIM(,))

  Note that the parameter lists contain only data type and di-
  mension information; they cannot contain any format or
  actual parameters. When the external procedure is in-
  voked, BASIC ensures that the actual parameter data type
  matches the data type specified in the EXTERNAL declara-
  tion. However, BASIC does not check to make sure that the
  parameters declared in the EXTERNAL statement match
  those in the external routine. You must ensure that these
  parameters match.

  You can pass data of any BASIC data type to a BASIC sub-
  program, including RFAs and RECORDs. BASIC allows you
  to pass up to 255 parameters, separated by commas. The
  data can be any one of the following:

  .
        Constants
  .
        Variables
  .
        Expressions
  .
        Functions
  .
        Array elements
  .
        Entire arrays (but not virtual arrays)

  For passing constants, variables, functions, and array ele-
  ments, name them in the argument list. For example:
  CALL SUB01(var1, var2)
  CALL SUB02(Po_num%, Vouch, 66.67, Cust_list(5), FNA(B%))

  However, when passing an entire array, you must use a
  special format. You specify the array name followed by com-
  mas enclosed in parentheses. The number of commas must
  be the number of array dimensions minus one. For ex-
  ample, array_name( ) is a one-dimensional array, array_
  name(,) is a two-dimensional array, array_name(,,) is a
  three-dimensional array, and so on.

  The following example creates a three-dimensional array,
  loads the array with values, and passes the array to a sub-
  program as a parameter. The subprogram can access and
  change values in array elements, and these changes remain
  in effect when control returns to the main program.
  EXAMPLE: Click to display example.

  If you do not specify data types for parameters, the default
  data type is determined by:

  .
        The last specified parameter data type
  .
        An OPTION statement
  .
        A BASIC compilation qualifier (for example, /REAL_
        SIZE=DOUBLE)
  .
        The system default

  The last specified parameter data type overrides all the other
  default data types, the defaults specified in the OPTION
  statement override any compilation qualifiers and system de-
  faults, and so on. See Chapter 2 for more information about
  the OPTION statement and establishing data-type defaults.

  When you know the length of a string or the dimensions of
  an array at compile time, you can achieve optimum perfor-
  mance by passing them BY REF. When you call programs
  written in other languages, the practice of declaring subpro-
  grams and specifying the data types of parameters becomes
  more important because other languages may not use the
  BASIC default parameter-passing mechanisms. For more
  information about calling subprograms written in other
  languages, see Chapter 20.