Investigate bug with multiply imputed data and ```ls.mixed```

[whatever]

ls.mixed gets an error while computing the design matrix within its qi method. This seems to only occur for multiply imputed models.

Program output and a traceback can be found here:

>>>> model.full <- zelig(attitude ~ age + married + male + income + tag(1 |
>>>> state), model = "ls.mixed", data = imp$imputations,  REML = FALSE,
>>>> subset = wave4)
>>>>
>>>> x.out <- setx(model.full)
>>>>
>>>> simout <- sim(x.out, x = x.out)
>>>>

The model estimates fine with the zelig() function, setx() works fine in setting the explanatory variables at their means, but when I attempt to use sim(), I get:

>>>>
>>>> Error in eval(expr, envir, enclos) : object 'age' not found
>>>>
>>>> A traceback produces this:
>>>>
>>>> 13: eval(expr, envir, enclos)
>>>> 12: eval(predvars, data, env)
>>>> 11: model.frame.default(object, data, xlev = xlev)
>>>> 10: model.frame(object, data, xlev = xlev)
>>>> 9: model.matrix.default(parsefml$fixed, data = D)
>>>> 8: model.matrix(parsefml$fixed, data = D)
>>>> 7: model.matrix(parsefml$fixed, data = D)
>>>> 6: is.data.frame(x)
>>>> 5: colnames(model.matrix(parsefml$fixed, data = D))
>>>> 4: qi.mer(object[[1]], simpar = simpar, x = as.matrix(x), x1 = if
>>>> (!is.null(x1)) as.matrix(x1))
>>>> 3: qi(object[[1]], simpar = simpar, x = as.matrix(x), x1 = if
>>>> (!is.null(x1)) as.matrix(x1))
>>>> 2: sim.setx.MI(model.full, x = x.out)
>>>> 1: sim(model.full, x = x.out)

[cchoirat]

Add an example in Zelig 5.