_G_e_n_e_r_a_t_e _F_a_c_t_o_r _L_e_v_e_l_s
	
	     gl(n, k, length, labels=1:n, ordered=FALSE)
	
	_D_e_s_c_r_i_p_t_i_o_n:
	
	     gl generates factors by specifying the pattern of their
	     levels.  The result has levels from 1 to n with each
	     value replicated in groups of length k out to a total
	     length of length.  Labels for the resulting factor lev-
	     els can be optionally specified with the arguments
	     labels and the factor levels can be made ordered by
	     specifying ordered=TRUE.  gl is modelled on the GLIM
	     function of the same name.
	
	_E_x_a_m_p_l_e_s:
	
	     # 20 alternating 1s and 2s
	     gl(2, 1, 20)
	
	     # alternating pairs of 1s and 2s
	     gl(2, 2, 20)