Redefine an ill-defined variable in the repl

[xnning]

7b09688

With the above commit, is there a way that I can redefine an ill-defined variable in the repl? say

>=> x = 1
>=> y = x' + 1 -- oops I got a typo. error: variable not in scope: x'. That's fine
>=> y = x + 1 -- would like to correct it, but now: variable already defined: y

[apaszke]

Ah good catch! I agree that a well-typed definition of a variable should shadow an incorrectly defined variable!