Assignment causes loop and then NaN

[GreenHapi]

Ubuntu 16.10, elm-repl 0.17.0 and elm-repl 0.17.1

> x = 1
1 : number
> y = x + 2
3 : number
> x
1 : number
> y
3 : number
> x = y
> x
> y
NaN : number

I'm expecting some kind of error message when I'm typing x = y (assignment causes loop).

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

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