Genuine copy of an element from an associative array

[Holzofen via Digitalmars-d-learn]

This is my (feeble) source code, for testing purposes only:
    uint64 sod_decompose(const uint64 n, const uint64 mod, const 
ref uint64[] primes, const ref uint64[uint64][uint64] factorials)
    {
        auto result = factorials[n];

        for (uint64 k = 2; k < n - 1; k++) {
            auto backup = factorials[n];
            tuple("1", backup).writeln;
            foreach (p, e; factorials[k]) {
                backup[p] -= e;
            }
            tuple("2", backup).writeln;
            foreach (p, e; factorials[n-k]) {
                backup[p] -= e;
            }
            tuple("3", backup).writeln;
        }

        return 0;
    }

uint64 is equivalent to ulong. Upon compilation I get this error:
Error: cannot modify `const` expression `backup[p]`
which concerns these two lines: backup[p] -= e;

Without the const statement in the function declaration the 
source array will be modified which is definitely not desirable. 
Could anybody point me to a way how to make a straight copy 
without meddling with the source array?

Thank you!