Process substitution

def process_sub(ref, alt_str):
    """Process substitution"""
    if len(ref) == len(alt_str):
        if len(ref) == 1:
            return record.Substitution(record.SNV, alt_str)
        else:
            return record.Substitution(record.MNV, alt_str)
    elif len(ref) > len(alt_str):
        return process_sub_grow(ref, alt_str)
    else:  # len(ref) < len(alt_str):
        return process_sub_shrink(ref, alt_str)