11.2.1 Single-Line DEF Functions
  In a single-line DEF, the function name, the formal pa-
  rameter list, and the defining expression all appear on the
  same line. The defining expression specifies the calculations
  that the function performs. You can pass up to 255 argu-
  ments to this function through the formal parameter list.
  These parameters are variables local to the function defini-
  tion, and each formal parameter can be preceded by a data
  type keyword.

  The following example creates a function named fnra-
  tio . This function has two formal parameters: numer and
  denomin , whose ratio is returned as a REAL value.

  When the function is invoked, BASIC does the following:

  .
        Copies the values 5.6 and 7.8 into the formal parameters
        numer and denomin
  .
        Evaluates the expression to the right of the equal sign
  .
        Returns the value to the statement that invoked the
        function (the PRINT statement)

  The PRINT statement then prints the returned value.
  DEF REAL fnratio (numer, denomin) = numer / denomin
  PRINT fnratio(5.6, 7.8)
  END

  Output
    .717949
  Note that the actual parameters you supply must agree in
  number and data type with those in the formal parameter
  list; you must supply numeric values for numeric variables,
  and string values for string variables.

  The defining expression for a single-line function definition
  can contain any constant, variable, BASIC built-in func-
  tion, or any user-defined function except the function being
  defined. The following examples are valid function definitions:
  DEF FN_A(X) = X^2 + 3 * X + 4
  DEF FN_B(X) = FN_A(X) / 2 + FN_A(X)
  DEF FN_C(X) = SQR(X+4) + 1
  DEF CUBE(X) = X ^ 3

  Note that the name of the last function defined does not begin
  with FN. This is valid as long as no reference to the function
  lexically precedes the function definition.

  You can also define a function that has no formal param-
  eters. The following function definition uses three BASIC
  built-in functions to return an integer corresponding to the
  day of the month:

  .
        DATE$( 0 ) returns a date string in the form dd-Mmm-
        yy .
  .
        The SEG$ function strips out of this value the characters
        starting at character position 1 up to and including the
        character at position 2 (the day number).
  .
        The VAL% function converts this resulting numeric
        string to an integer. In this way, fnday_number returns
        the day of the month as an integer.
  DEF INTEGER fnday_number = VAL% (SEG$(DATE$(0%), 1%, 2%))