refine mkProb for + case.  Fix bug in one call to mkProb.  Call mkProb on some things known to be of type Prob to fix up powering.
refine mkProb for + case.  Fix bug in one call to mkProb.  Call mkProb on some things known to be of type Prob to fix up powering.

zaxtax/hakaru,bitemyapp/hakaru,zachsully/hakaru,zaxtax/hakaru,suhailshergill/hakaru,zachsully/hakaru