[boost] What is a secret of mpl::set?

I have implemented typeof+mpl::set based "unique" algorithm for overloads library. MPL associative containers are impressive, I must say. On an overload set containing 200 functions with unique signatures at positions 1, 2, 10, 39 and 200 new version compiles in 12 seconds on my notebook while for old version it's 47 seconds. What is a secret of mpl::set? I expected it's typeof-based like "at" for vectors, but it seems it's not. If it doesn't use typeof, may be I could try to put mpl::set idea into my algorithm? -- Alexander Nasonov