A filter primitive that translates between double-precision
    numbers and their external representations.

    This routine is implemented by four XDR routines:
    xdr_       Converts VAX D-format floating-point numbers.
    double_D
    xdr_       Converts VAX G-format floating-point numbers.
    double_G
    xdr_       Converts IEEE T-format floating-point numbers.
    double_T
    xdr_       Converts IEEE X-format floating-point numbers.
    double_X
    You can reference these routines explicitly or you can use
    compiler settings to control which routine is used when you
    reference the xdr_double routine.