DDFUN: A double-double package with special functions (approximately 30 digit
precision).

This package permits one to perform floating-point computations (real and
complex) to double-double precision (approximately 30 digits), by making only
relatively minor changes to existing Fortran programs. It is intended as a
substitute for IEEE 128-bit quad precision on systems where quad precision is
not available. All basic arithmetic operations and transcendental functions are
supported, together with numerous special functions. The package should run
correctly on any Unix-based system supporting a Fortran-2008 compiler and IEEE
64-bit floating-point arithmetic.