def is_vcf(fname):
    """Convenience method to check if a file is a vcf file.

    >>> is_vcf("./foo")
    False
    >>> is_vcf("./foo.vcf")
    True
    >>> is_vcf("./foo.vcf.GZ")
    True
    """
    return fname.lower().endswith('.vcf') or fname.lower().endswith('.vcf.gz')
