Character. Specifies the column to choose:
"nt" chooses the CDR3 nucleotide column,
"aa" chooses the CDR3 amino acid column,
"v" chooses the V gene segment column,
"j" chooses the J gene segment column.
.col
A string that specifies the column(s) to be processed. Pass one of the
following strings, separated by the plus sign: "nt" for nucleotide sequences,
"aa" for amino acid sequences, "v" for V gene segments, "j" for J gene segments.