A1_to_R1C1("$A$1")
A1_to_R1C1("A1") ## raises a warning, returns NA
A1_to_R1C1("A1", strict = FALSE) ## unless strict = FALSE
A1_to_R1C1(c("A1", "B$4")) ## raises a warning, includes an NA, because
A1_to_R1C1(c("A1", "B$4"), strict = FALSE) ## mixed ref always returns NA
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