Craig,
This blog post was written for Stata 14.1. In Stata 13.1, mlexp could not include a constant in an equation.
This is how a probit regression was coded in mlexp previously
sysuse auto
mlexp (ln(cond(foreign==1, ///
normal({xb:gear_ratio turn} + {c}), ///
normal(-1*({xb:} +{c})))))
The {c} parameter represents the constant.
In Stata 14.1, we would code
sysuse auto
mlexp (ln(cond(foreign==1, ///
normal({xb:gear_ratio turn _cons}), ///
normal(-1*({xb:})))))