def norm(x1, x2=None):
    """
Normalizes x1. If also given as input x2, then normalizes x1 to x2.

:param x1: input array
:param x2: optional
:returns: normalized x1
	"""
    if x2 is None:
        return x1 / x1.max()
    else:
        return x1 * x2.max() / x1.max()
