一个基于stackengine的方案为:
\documentclass{article}
\usepackage{centernot}
\usepackage{stackengine}
\usepackage{amsmath,amssymb}
\newcommand*{\notLongrightarrow}{\centernot{\Longrightarrow}}
\newcommand*{\notLongleftarrow}{\centernot{\Longleftarrow}}
\newcommand*{\onlyRtoL}[1][-2pt]{\mathrel{\stackanchor[#1]{$\Longleftarrow$}{$\notLongrightarrow$}}}
\begin{document}
\[A \Longrightarrow B \notLongrightarrow C\]
\[A \Longleftarrow B \notLongleftarrow C\]
\[A \mathrel{\stackanchor{$\Longleftarrow$}{$\Longrightarrow$}} B \]
\[A \mathrel{\stackanchor[4pt]{$\notLongleftarrow$}{$\Longrightarrow$}} B \]
\[
\lim_{n\to\infty} y_n \quad \onlyRtoL[5pt] \quad |y_1|+|y_2|+\cdots+|y_n|\leq M\]
\end{document}



























问 纵向堆叠箭头符号的最佳实践?