_M_a_t_r_i_x _C_r_o_s_s_p_r_o_d_u_c_t
	
	     crossprod(x, y=x)
	
	_D_e_s_c_r_i_p_t_i_o_n:
	
	     Given matrices x and y as arguments, t returns their
	     matrix cross-product.  This is formally equivalent to,
	     but faster than, the call t(x) %*% y.