add findpackage

mkessler-findpackage.sty

@@ -0,0 +1,48 @@
+\NeedsTeXFormat{LaTeX2e}
+\ProvidesPackage{mkessler-findpackage}[2021-09-06 - Find a package that provides some command]
+
+
+%%%%
+% This is just a quick packaging of the post by
+%   'Martin Nyolt' (https://tex.stackexchange.com/users/78783/martin-nyolt)
+% found at
+%   https://tex.stackexchange.com/users/78783/martin-nyolt
+% Note that SO answers are by default CC BY-SA 3.0 licensed, see
+%   https://creativecommons.org/licenses/by-sa/3.0/
+
+\def\find@load#1[#2]#3[#4]{%
+    % load the package
+    \find@fileswith@pti@ns{#1}[#2]{#3}[#4]%
+    % check if commands exists now
+    \ifcsname\find@command\endcsname
+        \typeout{Package #3 introduces command \find@command.}%
+        % command has been found, revert to original version without checks
+        \let\@fileswith@pti@ns\find@fileswith@pti@ns
+    \else
+        % somehow, \@fileswith@pti@ns is restored after loading a package
+        % thus, patch it again
+        \let\@fileswith@pti@ns\find@load
+    \fi
+}%
+
+\newcommand*{\findpackagebycommand}[1]{%
+    % using this multiple times - esp. when the package has not been found yet -
+    % will break things. Thus, check first that \find@command has never been defined before
+    \ifx\find@command\undefined
+        \def\find@command{#1}%
+        % first, check if this command is already defined
+        \ifcsname\find@command\endcsname
+            % in this case, just issue a warning and do nothing
+            \@latex@warning@no@line{Command \find@command \space is already defined}%
+        \else
+            % overwrite the internal \@fileswith@pti@ns command, which does the actual loading
+            % \@fileswith@pti@ns is used internally by \usepackage and \RequirePackage
+            \let\find@fileswith@pti@ns\@fileswith@pti@ns
+            \let\@fileswith@pti@ns\find@load
+        \fi
+    \else
+        % used multiple times - prevent and give a warning
+        \@latex@warning@no@line{You can use \protect\findpackagebycommand \space only once}%
+        \errmessage{Invalid use of command findpackagebycommand.}%
+    \fi
+}