Add adds x and y.

NewDecFromString returns a new Dec from a string It only support finite numbers, not NaN, +Inf, -Inf.

SubNonNegative subtracts the value of y from x and returns the result with arbitrary precision.

Dec is a wrapper struct around apd.Decimal that does no mutation of apd.Decimal's when performing arithmetic, instead creating a new apd.Decimal for every operation ensuring usage is safe.