Fix nested CSE

[spcornelius]

Fixes #568.

[github-actions]

Benchmark Results

	master	07fdc5f...	master/07fdc5f6c1bfc5...
overhead/acrule/a+2	0.743 ± 0.023 μs	0.744 ± 0.025 μs	0.999
overhead/acrule/a+2+b	0.736 ± 0.024 μs	0.736 ± 0.024 μs	1
overhead/acrule/a+b	0.259 ± 0.012 μs	0.256 ± 0.0075 μs	1.01
overhead/acrule/noop:Int	25.9 ± 0.92 ns	26.2 ± 0.93 ns	0.988
overhead/acrule/noop:Sym	0.0341 ± 0.0054 μs	0.0333 ± 0.0057 μs	1.02
overhead/rule/noop:Int	0.037 ± 0.00037 μs	0.0371 ± 0.001 μs	0.996
overhead/rule/noop:Sym	0.0434 ± 0.002 μs	0.0428 ± 0.0017 μs	1.01
overhead/rule/noop:Term	0.0437 ± 0.0017 μs	0.0432 ± 0.0021 μs	1.01
overhead/ruleset/noop:Int	0.131 ± 0.007 μs	0.125 ± 0.0031 μs	1.05
overhead/ruleset/noop:Sym	0.134 ± 0.0039 μs	0.137 ± 0.0036 μs	0.979
overhead/ruleset/noop:Term	6.79 ± 0.49 μs	6.86 ± 0.47 μs	0.99
overhead/simplify/noop:Int	0.136 ± 0.0039 μs	0.151 ± 0.0043 μs	0.901
overhead/simplify/noop:Sym	0.152 ± 0.0046 μs	0.157 ± 0.0033 μs	0.967
overhead/simplify/noop:Term	0.0459 ± 0.0029 ms	0.0442 ± 0.0024 ms	1.04
overhead/simplify/randterm (+, *):serial	0.136 ± 0.0027 s	0.132 ± 0.0029 s	1.03
overhead/simplify/randterm (+, *):thread	0.0846 ± 0.023 s	0.082 ± 0.024 s	1.03
overhead/simplify/randterm (/, *):serial	0.277 ± 0.0098 ms	0.263 ± 0.0094 ms	1.05
overhead/simplify/randterm (/, *):thread	0.339 ± 0.012 ms	0.314 ± 0.011 ms	1.08
overhead/substitute/a	0.122 ± 0.0031 ms	0.125 ± 0.0031 ms	0.983
overhead/substitute/a,b	0.101 ± 0.0028 ms	0.107 ± 0.003 ms	0.945
overhead/substitute/a,b,c	16.9 ± 0.75 μs	18.3 ± 0.79 μs	0.923
polyform/easy_iszero	0.0525 ± 0.0024 ms	0.0536 ± 0.0024 ms	0.979
polyform/isone	2.79 ± 0.01 ns	3.1 ± 0.01 ns	0.9
polyform/iszero	5.59 ± 0.092 ms	5.62 ± 0.086 ms	0.995
polyform/simplify_fractions	3.39 ± 0.069 ms	3.4 ± 0.065 ms	0.997
time_to_load	4.61 ± 0.019 s	4.64 ± 0.023 s	0.995

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).