Deal with error in the correct way

[zenna]

Consider the following program

X = normal(0,1)
Y = (1, X)

X can be negative and negative values are not valid values for the standard deviation of Y. There seem to be a few different plausible approaches

1. Make this an invalid program
2. Only consider the valid programs
3. Treat both the valid and invalid programs as possible conditions that we can query about,