docs: def gcd( *a ):
    """Greatest common divisor.
  
    Usage: gcd( [ 2, 4, 6 ] )
    or:    gcd( 2, 4, 6 )
    """
  
    if len( a ) > 1: return reduce( gcd2, a )
    if hasattr( a[0], "__iter__" ): return reduce( gcd2, a[0] )
    return a[0]