Calculates the value of input according to a specified modulus,
       using double precision floating values.

1  –  Input

       This function takes two parameters: a signed number and a
       modulus. The two must be separated by a comma and enclosed in
       parentheses.

2  –  Output

       A real number of type G-floating.

3  –  Example

       DTR> PRINT FN$MOD (31,7)

         FN$MOD

        3.0000E+00

       DTR>