I have used Steven B. Segletes's answer for many year and value it. Unfortunately, it is quite computationally expensive, and made some simple document that I had take 26 seconds to compile.
I have re-written my own solution, without the double-lined (d) variations: this is not exactly an answer to this question, but I thought it could be helpful nevertheless.
\documentclass{article}\usepackage{tikz}\usepackage{mathtools}\usetikzlibrary{calc,decorations.pathmorphing,shapes}\newcounter{sarrow}\NewDocumentCommand{\xrsquigarrow}{m}{ % Backward arrow \stepcounter{sarrow}% \mathrel{\begin{tikzpicture}[baseline= {( $(current bounding box.south)$ )}] \node[inner sep=.5ex, font=\scriptsize] (\thesarrow) {$#1$}; \path[draw,->,decorate, %decoration={zigzag,amplitude=0.7pt,segment length=1.2mm,pre=lineto,pre length=4pt} decoration={snake, amplitude=-1pt, segment length =1.5mm, pre=lineto, post length=2pt%, post length=3pt } ] (\thesarrow.south west) -- (\thesarrow.south east); \end{tikzpicture}}%}\newcommand{\tootip}{\mathrel{\tikz[anchor=base]{\draw[->](0,0)--(.01,0);}}} % https://tex.stackexchange.com/a/645771/34551% https://tex.stackexchange.com/a/260581/34551\newcommand{\rightarrowdbl}{\mathrel{\ooalign{ % https://tex.stackexchange.com/a/22375/34551 $\rightarrow$\cr % \hidewidth\raise.23em\hbox{$\tootip\mkern4mu$}}} }\newcommand{\xrightarrowdbl}[1]{% \mathrel{\ooalign{% $\xrightarrow{#1}$\cr% \hidewidth\raise.23em\hbox{$\tootip\mkern4mu$}}}}\ExplSyntaxOn\RenewDocumentCommand{\r}{m o o} % m = direction (b, f, bf)% o = label% o = key% Assumption is that you will never have a key without a label.{ \str_case:nnF { #1 } { {f}{% Forward case \IfValueTF{#2} % We test if the label is here. { \IfValueTF{#3}% We test if the key is here. {\xrightarrow{#2[#3]} } % Forward, label and key {\xrightarrow{#2}} % Forward, a label but no key } { \rightarrow } % Forward, no label nor key } {b}{% backward case \IfValueTF{#2} % We test if the label is here. { \IfValueTF{#3}% We test if the key is here. {\xrsquigarrow{#2[#3]}} % Backward, label and key {\xrsquigarrow{#2}} % Backward, a label but no key } { \xrsquigarrow{\mkern10mu} } % Backward, no label nor key } {fb}{ % Forward and backward case \IfValueTF{#2} % We test if the label is here. { \IfValueTF{#3}% We test if the key is here. { \xrightarrowdbl{#2[#3]}} % Backward, label and key { \xrightarrowdbl{#2}} % Backward, a label but no key } {\rightarrowdbl} % Forward and backward, no label nor key } } { #1 }}\ExplSyntaxOff\begin{document}\begin{align*} \Aboxed{P \r{f} Q} & \Aboxed{P \r{f}[\alpha] Q} & \Aboxed{P \r{f}[\alpha][k] Q}\\ \Aboxed{P \r{b} Q} & \Aboxed{P \r{b}[\alpha] Q} & \Aboxed{P \r{b}[\alpha][k] Q} \\ \Aboxed{P \r{fb} Q} & \Aboxed{P \r{fb}[\alpha] Q} & \Aboxed{P \r{fb}[\alpha][k] Q} \\\end{align*}\end{document}gives:
My document went from 26 seconds to 6 seconds using this alternative way of generating arrows…