Fix head & arguments accessor functions

[shashi]

This is a change intended in 2.0 but apparently never got tested.

[github-actions]

Benchmark Results

	master	b90aad0...	master/b90aad0a5067ef...
overhead/acrule/a+2	0.751 ± 0.02 μs	0.721 ± 0.017 μs	1.04
overhead/acrule/a+2+b	0.734 ± 0.02 μs	0.701 ± 0.018 μs	1.05
overhead/acrule/a+b	0.26 ± 0.0076 μs	0.257 ± 0.0062 μs	1.01
overhead/acrule/noop:Int	25.3 ± 0.05 ns	25 ± 0.05 ns	1.01
overhead/acrule/noop:Sym	0.0368 ± 0.0056 μs	0.0365 ± 0.0057 μs	1.01
overhead/rule/noop:Int	0.0447 ± 0.00045 μs	0.0444 ± 0.0005 μs	1.01
overhead/rule/noop:Sym	0.0558 ± 0.0028 μs	0.056 ± 0.0023 μs	0.997
overhead/rule/noop:Term	0.0566 ± 0.0027 μs	0.0554 ± 0.0023 μs	1.02
overhead/ruleset/noop:Int	0.131 ± 0.0036 μs	0.134 ± 0.0036 μs	0.979
overhead/ruleset/noop:Sym	0.148 ± 0.0057 μs	0.146 ± 0.0052 μs	1.01
overhead/ruleset/noop:Term	3.81 ± 0.2 μs	3.77 ± 0.16 μs	1.01
overhead/simplify/noop:Int	0.147 ± 0.0019 μs	0.14 ± 0.0015 μs	1.05
overhead/simplify/noop:Sym	0.159 ± 0.0029 μs	0.153 ± 0.004 μs	1.04
overhead/simplify/noop:Term	0.0398 ± 0.0022 ms	0.0378 ± 0.0025 ms	1.05
overhead/simplify/randterm (+, *):serial	0.122 ± 0.0039 s	0.123 ± 0.0056 s	0.996
overhead/simplify/randterm (+, *):thread	0.0731 ± 0.025 s	0.0766 ± 0.026 s	0.954
overhead/simplify/randterm (/, *):serial	0.237 ± 0.0076 ms	0.231 ± 0.008 ms	1.03
overhead/simplify/randterm (/, *):thread	0.273 ± 0.0087 ms	0.265 ± 0.0087 ms	1.03
overhead/substitute/a	0.0574 ± 0.0017 ms	0.0561 ± 0.0016 ms	1.02
overhead/substitute/a,b	0.0499 ± 0.0015 ms	0.0499 ± 0.0015 ms	1
overhead/substitute/a,b,c	16.9 ± 0.84 μs	17 ± 0.74 μs	0.995
polyform/easy_iszero	0.0343 ± 0.002 ms	0.0337 ± 0.002 ms	1.02
polyform/isone	2.79 ± 0.01 ns	2.79 ± 0.01 ns	1
polyform/iszero	1.83 ± 0.036 ms	1.82 ± 0.041 ms	1.01
polyform/simplify_fractions	2.58 ± 0.053 ms	2.54 ± 0.049 ms	1.01
time_to_load	4.57 ± 0.04 s	4.59 ± 0.018 s	0.996

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).