What does this mean?

[nbecker@fred.net]

In the following diagnostic, why does "(+rng)" mean?

TestSigGen.cc:30:   instantiated from here
SigGen.H:41: could not convert 
`uniform_real<boost::random::mersenne_twister<long unsigned int, 624, 
397, 31, -1727483681, 11, 7, -1658038656, 15, -272236544, 18, 
-948541730>,double>((+rng), 0x000000000000000000000000, 
0x000000c06821a2da0fc90140)' to `boost::uniform_real<rng_t, double> &'
SigGen.H:18: in passing argument 3 of `maybe_random<Rng>::maybe_random 
(bool, typename Rng::result_type, Rng &) [with Rng = 
boost::uniform_real<rng_t, double>]'