ddDiffDd
Returns the result of subtracting the second given double-double-precision floating point number from the first.
relative error bound: 3u^2 + 13u^3, i.e. fl(a-b) = (a-b)(1+ϵ), where ϵ <= 3u^2 + 13u^3, u = 0.5 * Number.EPSILON
the error bound is not sharp - the worst case that could be found by the authors were 2.25u^2
ALGORITHM 6 of https://hal.archives-ouvertes.fr/hal-01351529v3/document
Parameters
x
a double-double precision floating point number
y
another double-double precision floating point number