fp64_lib was extended to include the functions in section 5.3.1 of the IEEE Standard 754-2019:
fp64_nextUp(x) returns the least possible number that is just > x. Usually, the difference of nextUp(x)-x is 1ulp (unit in last place).
fp64_nextDown(x) returns the biggest possible number that is just < x.
See the documentation for some interesting examples.