Decimal.opBinary

immutable a = Decimal(123000, 3);
immutable b = Decimal(456, 3);
assert(a + b == Decimal(123456, 3));
assert(a - b == Decimal(122544, 3));

immutable c = Decimal(-456, 3);
assert(a + c == Decimal(122544, 3));
assert(a - c == Decimal(123456, 3));

assert(b - a == Decimal(-122544, 3));