_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)
	
	_V_a_l_u_e:
	
	     This function generates factors by specifying the pat-
	     tern 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 levels 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)