Rename first argument following tidyverse principles

[coveralls]

Pull Request Test Coverage Report for Build 691

• 108 of 122 (88.52%) changed or added relevant lines in 6 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage remained the same at 87.259%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
R/add_subquad.R	22	23	95.65%
R/pick_drop_var_cond.R	7	8	87.5%
R/pick_main_stem.R	18	19	94.74%
R/sanitize.R	3	4	75.0%
R/add_var.R	48	58	82.76%
<!--	Total:	108	122	88.52%	-->

Files with Coverage Reduction	New Missed Lines	%
R/sanitize.R	1	60.0%
R/pick_drop_var_cond.R	1	77.78%
<!--	Total:	2		-->

Totals
Change from base Build 687:	0.0%
Covered Lines:	589
Relevant Lines:	675

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💛 - Coveralls