GMP: Are self-assignments forbidden?

I'm using the GMP library for arbitrary precision in C. All usage examples that I've seen seem to avoid self assignments such as:

Syntax : void mpz_add (mpz_t rop, const mpz_t op1, const mpz_t op2)

mpz_add(a, a, b); // Assign a+b to a

Is this usage allowed or should I only resort to assigning to a third variable?

via Chebli Mohamed