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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sky2vec(cls, sky):
 """ Convert sky positions in to 3d-vectors on the unit sphere. Parameters sky : numpy.array Sky coordinates as an array of (ra,dec) Returns ------- vec : numpy.array Unit vectors as an array of (x,y,z) See Also -------- :func:`AegeanTools.regions.Region.vec2sky` """

        theta_phi = cls.sky2ang(sky)
        theta, phi = map(np.array, list(zip(*theta_phi)))
        vec = hp.ang2vec(theta, phi)
        return vec