(!) \lim_{n \in N} |x`_{p_n}-c|=0 \\
0 \leq \lim_{n \in N} |x``_{p_n}-x`_{p_n}| \leq \lim_{n \in N}1/n =0 
 \Rightarrow \lim_{n \in N} |x``_{p_n}-x`_{p_n}|=0 \\
\Rightarrow^{po\_prop\_o\_sumi\_limesa} \lim_{n \in N} [|x``_{p_n}-x`_{p_n}|+|x`_{p_n}-c| ] = 0