<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def sequence(self, fasta, use_strand=True):
 """ Retrieves the sequence of this feature as a string. Uses the pyfaidx package. Parameters fasta : str If str, then it's a FASTA-format filename; otherwise assume it's a pyfaidx.Fasta object. use_strand : bool If True (default), the sequence returned will be reverse-complemented for minus-strand features. Returns ------- string """

        if isinstance(fasta, six.string_types):
            fasta = Fasta(fasta, as_raw=False)

        # recall GTF/GFF is 1-based closed;  pyfaidx uses Python slice notation
        # and is therefore 0-based half-open.
        seq = fasta[self.chrom][self.start-1:self.stop]
        if use_strand and self.strand == '-':
            seq = seq.reverse.complement
        return seq.seq