
    library(AER)
    data(CollegeDistance)


    # completar la función `TSLS()`
    TSLS <- function(Y, X, W = NULL, Z, data) {
    fs_model <- lm(as.formula(paste(..., collapse = "+"))), data = data)
    X_fitted <- ...
    ss_model <- lm(as.formula(paste(..., paste(..., collapse = "+"))),  data = data)
    return(coefficients(...))}

    # use `TSLS()` para reproducir las estimaciones del ejercicio 3


    # completar la función `TSLS()`
    TSLS <- function(Y, X, W = NULL, Z, data) {

    # regresión de primera etapa y valores ajustados
    fs_model <- lm(as.formula(paste(X, "~", paste(c(Z, W), collapse = "+"))), data = data)
    X_fitted <- fs_model$fitted.values
  
    # regresión de segunda etapa
    ss_model <- lm(as.formula(paste(Y, "~", paste(W, collapse = "+"), "+ X_fitted")),  data = data)

    # coeficientes de retorno de la segunda etapa
    return(
      coefficients(ss_model)
    )}
     
    # use `TSLS()` para reproducir las estimaciones del ejercicio 3
    TSLS(Y = "log(wage)", X = "education", Z = "distance", data = CollegeDistance)
    TSLS(Y = "log(wage)", 
        X = "education", 
        W = c("unemp", "ethnicity", "gender", "urban"), 
        Z = "distance", 
        data = CollegeDistance)


    ex() %>% check_fun_def("TSLS") %>% {
      check_arguments(.)
      check_call(., Y = "log(wage)", X = "education", Z = "distance", data = CollegeDistance) %>% check_result %>% check_equal
    } 
    ex() %>% check_function("TSLS", index = 1) %>% {
      check_arg(., "Y") %>% check_equal()
      check_arg(., "X") %>% check_equal()
      check_arg(., "Z") %>% check_equal()
    } 
    ex() %>% check_function("TSLS", index = 2) %>% {
      check_arg(., "Y") %>% check_equal()
      check_arg(., "X") %>% check_equal()
      check_arg(., "W") %>% check_equal()
      check_arg(., "Z") %>% check_equal()
    } 
    success_msg("¡Bien hecho!")