subtract method
Implementation
UcumDecimal subtract(UcumDecimal other) {
UcumDecimal result;
if (negative && !other.negative) {
result = doAdd(other);
result.negative = true;
} else if (!negative && other.negative) {
result = doAdd(other);
} else if (negative && other.negative) {
result = doSubtract(other);
result.negative = !result.negative;
} else {
result = other.doSubtract(this);
result.negative = !result.negative;
}
return result;
}