_N_o_t _A_v_a_i_l_a_b_l_e _V_a_l_u_e
	
	     NA
	     is.na(x)
	     as.na(x)
	
	_D_e_s_c_r_i_p_t_i_o_n:
	
	     NA is a logical constant of length 1 which contains a
	     missing value indicator.  NA can be freely coerced to
	     any other vector type.
	
	     as.na ignores its argument and returns the value NA.
	
	     is.na takes a vector argument and returns a logical
	     vector of the same length containing TRUE for those
	     elements marked NA and FALSE otherwise.  dim, dimnames
	     and names attributes are preserved.
	
	_S_e_e _A_l_s_o:
	
	     complete.cases.