stan-dev/stan — pull request #943 by PeterLi2016 (2014-08-28 17:22:50+00:00)
Title: updated fvar so if value is NaN, then derivative is set to NaN

#### Summary:
Using unlikely() compiler hint, fvar now sets the derivative to NaN if the value is NaN.

#### Intended Effect:
Using unlikely() compiler hint, fvar now sets the derivative to NaN if the value is NaN.

#### How to Verify:
Run test/unit-agrad-fwd/fvar

#### Side Effects:
None.

#### Documentation:
None.

#### Reviewer Suggestions:
Anyone.