fix root check comparison

[GiggleLiu]

I wrote a tree, and this tree has a sophisticated method to compare the equivalence (to be concrete, it is a sum-product tree, I need to expand the expression).
When I print such a tree with millions of nodes, it freezes because of invoking the comparison.

This minor fix will make the root check more reliable and it solves my problem perfectly.

[codecov-commenter]

Codecov Report

Merging #92 (12d5a22) into master (84d2a28) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #92   +/-   ##
=======================================
  Coverage   68.31%   68.31%           
=======================================
  Files           7        7           
  Lines         404      404           
=======================================
  Hits          276      276           
  Misses        128      128           

Impacted Files	Coverage Δ
src/printing.jl	95.52% <100.00%> (ø)

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[GiggleLiu]

Hi, this is a one character fix, so it should be very easy to review. But this fix is quite important to my project, so it would be great if someone with write permission can help review this PR. Thanks! @jlumpe @Keno

[GiggleLiu]

Hello, is this repository still active?

[timholy]

My review queue is too large so I was hoping not to tackle this, but LGTM.

I'm going to run with this locally for a bit before releasing because changes like these can cause problems.