def peek_left_base(idl, genome, chr_prefixed):
    """Get the nucleotide immediately left to the indel
         
    Args:
        idl (Indel obj)
        genome (pysam.FastaFile): reference genome
        chr_prefixed (bool): True if chromosome names in BAM are "chr"-prefixed
    Returns:
        left_base (str)
    Example:
         pos:  0123  4567      
         ref:  ATGC  GCTA
         alt:  ATGCTAGCTA
         
         'TA' is inserted at 4.
         the left base is 'C' at 3
    """
    chr = idl.chr
    if not chr_prefixed:
        chr = chr.replace("chr", "")
    left_base = genome.fetch(chr, idl.pos - 2, idl.pos - 1)

    return left_base