Javadoc
Arithmetic Operations
There's no checking if an amount is of a different currency/issuer.
All operations return amounts of the same currency/issuer as the
first operand.
eg.
amountOne.add(amountTwo)
The currency/issuer of the resultant amount, is that of `amountOne`
Divide and multiply are equivalent to the javascript ripple-lib
ratio_human and product_human.