Big Number Calculator

This works on the decimal digits themselves rather than on floating point numbers, so every digit of the answer is right. It adds, subtracts, multiplies and divides whole numbers of up to 200 digits, returning a quotient and an exact remainder rather than a rounded decimal, and it also raises a number to a power and works out a factorial up to 1000 exactly. Each answer is checked before it is shown, division in full by multiplying the quotient back, and the rest by casting out nines. Whole numbers only, with a leading minus allowed and spaces or commas ignored. An ordinary calculator keeps about fifteen significant digits and quietly rounds the rest, which is the problem this one exists to avoid.