f <- function(x, y, ...) NULL
f_fm <- firmly(f, ~is.numeric, list(~x, ~y - x) ~ {. > 0})
identical(firm_core(f_fm), f) # [1] TRUE
firm_checks(f_fm) # 4 x 4 data frame
firm_error(f_fm) # [1] "simpleError"
firm_args(f_fm) # NULL
firm_args(firmly(f_fm, .warn_missing = "y")) # [1] "y"
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