_D_i_v_i_d_e _i_n_t_o _G_r_o_u_p_s
	
	     split(x, groups)
	
	_A_r_g_u_m_e_n_t_s:
	
	           x : vector containing the values to be divided
	               into groups.
	
	       group : a factor which defines the grouping.
	
	_D_e_s_c_r_i_p_t_i_o_n:
	
	     split divides the data in the vector x into the groups
	     defined by the factor groups.
	
	     The value returned is a list of vectors containing the
	     values for the groups.  The components of the list are
	     named by the factor levels of group.
	
	_E_x_a_m_p_l_e_s:
	
	     xa <- split(x, age)
	     lapply(xa, mean)
	     #split a matrix into a list by columns
	     split(x,col(x))