Given the min number of reads overlapping each nucleotide of
        a variant sequence, trim this sequence by getting rid of positions
        which are overlapped by fewer reads than specified.

def trim_by_coverage(self, min_reads):
        """
        Given the min number of reads overlapping each nucleotide of
        a variant sequence, trim this sequence by getting rid of positions
        which are overlapped by fewer reads than specified.
        """
        read_count_array = self.coverage()
        logger.info("Coverage: %s (len=%d)" % (
            read_count_array, len(read_count_array)))
        sufficient_coverage_mask = read_count_array >= min_reads
        sufficient_coverage_indices = np.argwhere(sufficient_coverage_mask)
        if len(sufficient_coverage_indices) == 0:
            logger.debug("No bases in %s have coverage >= %d" % (self, min_reads))
            return VariantSequence(prefix="", alt="", suffix="", reads=self.reads)
        variant_start_index, variant_end_index = self.variant_indices()
        # assuming that coverage drops off monotonically away from
        # variant nucleotides
        first_covered_index = sufficient_coverage_indices.min()
        last_covered_index = sufficient_coverage_indices.max()
        # adding 1 to last_covered_index since it's an inclusive index
        # whereas variant_end_index is the end of a half-open interval
        if (first_covered_index > variant_start_index or
                last_covered_index + 1 < variant_end_index):
            # Example:
            #   Nucleotide sequence:
            #       ACCCTTTT|AA|GGCGCGCC
            #   Coverage:
            #       12222333|44|33333211
            # Then the mask for bases covered >= 4x would be:
            #       ________|**|________
            # with indices:
            #       first_covered_index = 9
            #       last_covered_index = 10
            #       variant_start_index = 9
            #       variant_end_index = 11
            logger.debug("Some variant bases in %s don't have coverage >= %d" % (
                self, min_reads))
            return VariantSequence(prefix="", alt="", suffix="", reads=self.reads)
        return VariantSequence(
            prefix=self.prefix[first_covered_index:],
            alt=self.alt,
            suffix=self.suffix[:last_covered_index - variant_end_index + 1],
            reads=self.reads)