adding a second option for Phi
adding a second option for Phi

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math