_S_t_o_p _F_u_n_c_t_i_o_n _E_x_e_c_u_t_i_o_n
	
	     stop(message)
	
	_D_e_s_c_r_i_p_t_i_o_n:
	
	     stop stops execution of the current expression, prints
	     the message given as its argument and then returns to
	     the top level prompt.
	
	_S_e_e _A_l_s_o:
	
	     warning.
	
	_E_x_a_m_p_l_e_s:
	
	     if(iter > 10) stop("too many iterations")