x <- c(NA, 0, 1, 0.5, 0, NA, 99)
chk_blank(x) # Blank
chk_equals(x, 0) # Either blank or 0
chk_values(x, 0, 1) # Either blank, 0, 1, or 99
chk_range(x, 0, 1) # Either blank or in [0,1]
chk_range(x, 0, 1, 99) # Either blank, in [0,1], or equal to 99
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