Allow setting default variables in printing

[MilesCranmer]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 5733931712

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.94%

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Totals
Change from base Build 5733505867:	0.0%
Covered Lines:	1064
Relevant Lines:	1170

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💛 - Coveralls

[github-actions]

Benchmark Results

	master	082d5b9...	t[master]/t[082d5b9...]
eval/ComplexF32/evaluation	13.3 ± 0.92 ms	13.2 ± 0.9 ms	1.01
eval/ComplexF64/evaluation	15.6 ± 1.3 ms	15.9 ± 1.3 ms	0.98
eval/Float32/derivative	20.8 ± 3.9 ms	20.4 ± 3.1 ms	1.02
eval/Float32/derivative_turbo	22.8 ± 3.9 ms	21.7 ± 3.1 ms	1.05
eval/Float32/evaluation	4.16 ± 0.34 ms	4.05 ± 0.33 ms	1.03
eval/Float32/evaluation_turbo	1.28 ± 0.11 ms	1.25 ± 0.11 ms	1.02
eval/Float64/derivative	29.2 ± 0.96 ms	28.4 ± 1.3 ms	1.03
eval/Float64/derivative_turbo	0.0326 ± 0.0017 s	31.2 ± 1.2 ms	1.04
eval/Float64/evaluation	4.3 ± 0.35 ms	4.14 ± 0.4 ms	1.04
eval/Float64/evaluation_turbo	1.82 ± 0.15 ms	1.73 ± 0.15 ms	1.05
time_to_load	1.16 ± 0.044 s	1.11 ± 0.067 s	1.05
utils/combine_operators	0.0731 ± 0.0065 ms	0.0752 ± 0.0062 ms	0.972
utils/convert/break_sharing	0.0569 ± 0.0051 ms	0.0596 ± 0.007 ms	0.955
utils/convert/preserve_sharing	0.279 ± 0.015 ms	0.284 ± 0.014 ms	0.983
utils/copy/break_sharing	0.0564 ± 0.0035 ms	0.0578 ± 0.0051 ms	0.976
utils/copy/preserve_sharing	0.276 ± 0.0059 ms	0.278 ± 0.013 ms	0.992
utils/count_constants	0.0322 ± 0.0014 ms	31.3 ± 1.3 μs	1.03
utils/count_depth	0.0339 ± 0.0011 ms	0.033 ± 0.001 ms	1.03
utils/count_nodes	29.6 ± 1.8 μs	29.5 ± 1.8 μs	1
utils/get_set_constants!	0.126 ± 0.003 ms	0.126 ± 0.0088 ms	1
utils/has_constants	9.61 ± 0.94 μs	10.9 ± 1 μs	0.882
utils/has_operators	2.92 ± 0.21 μs	2.73 ± 0.067 μs	1.07
utils/index_constants	0.0961 ± 0.0084 ms	0.0955 ± 0.0048 ms	1.01
utils/is_constant	10.9 ± 1.1 μs	11.3 ± 1.1 μs	0.966
utils/simplify_tree	0.21 ± 0.025 ms	0.216 ± 0.022 ms	0.974
utils/string_tree	0.396 ± 0.014 ms	0.402 ± 0.012 ms	0.984

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).